Here is an extensive quote of the relevant information of our common law regarding use of deadly force in self defense.  It is from This document.

Obviously, this doesn't constitute legal advice from me, as I'm not a lawyer, and am merely quoting a document.

Self-defense

·    Most important of the justifications.

·    Nothing controversial about self-defense itself, but it comes up in controversial and sensitive contexts.

A victim can use non-deadly force any time that victim reasonably believes that force is about to be used on him.

Common Law Elements for Use of Deadly Force in Self-defense

(1) unlawful threatener, i.e. aggressor is wrongdoer and you are innocent agent;

(2) honest and reasonabe belief that you are subject to imminent harm, (reasonable because you need to be innocent and if not reasonable then negligent; imminent because if not you have a chance to get away. Subjective approach would allow everyone to claim justification at all times.);

(3) force used must be proportionate (because the balance of evils must be positive)

(4) Actor is not the original aggressor.

Classic common law says if you are initial provoker you lose self-defense unless you communicate withdrawal.

(5) No retreat requirement in most jurisdictions.

Where retreat requirement exists it only holds if actor knows of a place to which he could retreat in complete safety.

MPC Elements for Use of Deadly Force in Self-defense

(1) Force is immediately necessary to protect against unlawful force by another on the present occassion.

(2) Unlawful force threated is death, serious bodily injury, kidnapping or rape.

(3) Actor did not initially provoke for the purpose of using deadly force.

If you provoked with the use of non-deadly force and the other party responds with deadly force, MPC allows justified use of deadly force. (Still liable for initial battery.)

(4) Duty to retreat if actor knows he can avoid the need to use deadly force with complete safety to himself.  Minority Rule.  Three exceptions: don’t have to retreat from home, or if victim of rape or robbery, or if you’re a cop.

(5) Belief must be honest.

(6) Estimate of using force must not be negligent or reckless, if it is guilty of reckless or negligent crime.

Texas version of the Castle Law (copied from here):

A BILL TO BE ENTITLED
 AN ACT
 relating to the use of force or deadly force in defense of a person.
        BE IT ENACTED BY THE LEGISLATURE OF THE STATE OF TEXAS:
        SECTION 1.  Section 9.01, Penal Code, is amended by adding 
 Subdivisions (4) and (5) to read as follows:
              (4)  "Habitation" has the meaning assigned by Section 
 30.01.
              (5)  "Vehicle" has the meaning assigned by Section 
 30.01.
        SECTION 2.  Section 9.31, Penal Code, is amended by amending 
 Subsection (a) and adding Subsections (e) and (f) to read as 
 follows:
        (a)  Except as provided in Subsection (b), a person is 
 justified in using force against another when and to the degree the 
 actor [he] reasonably believes the force is immediately necessary 
 to protect the actor [himself] against the other's use or attempted 
 use of unlawful force.  The actor's belief that the force was 
 immediately necessary as described by this subsection is presumed 
 to be reasonable if the actor knew or had reason to believe that the 
 person against whom the force was used:
              (1)  unlawfully entered, or was attempting to enter 
 unlawfully, the actor's habitation, vehicle, or place of business 
 or employment;
              (2)  unlawfully removed, or was attempting to remove 
 unlawfully, the actor from the actor's habitation, vehicle, or 
 place of business or employment; or
              (3)  was committing or attempting to commit aggravated 
 kidnapping, murder, sexual assault, aggravated sexual assault, 
 robbery, or aggravated robbery.
        (e)  A person who has a right to be present at the location 
 where the force is used, who has not provoked the person against 
 whom the force is used, and who is not engaged in criminal activity 
 at the time the force is used is not required to retreat before 
 using force as described by this section.
        (f)  For purposes of Subsection (a), in determining whether 
 an actor described by Subsection (e) reasonably believed that the 
 use of force was necessary, a finder of fact may not consider 
 whether the actor failed to retreat.
        SECTION 3.  Section 9.32, Penal Code, is amended to read as 
 follows:
        Sec. 9.32.  DEADLY FORCE IN DEFENSE OF PERSON.  (a)  A person 
 is justified in using deadly force against another:
              (1)  if the actor [he] would be justified in using force 
 against the other under Section 9.31; and
              (2)  [if a reasonable person in the actor's situation 
 would not have retreated; and 
              [(3)]  when and to the degree the actor [he] reasonably 
 believes the deadly force is immediately necessary:
                    (A)  to protect the actor [himself] against the 
 other's use or attempted use of unlawful deadly force; or
                    (B)  to prevent the other's imminent commission of 
 aggravated kidnapping, murder, sexual assault, aggravated sexual 
 assault, robbery, or aggravated robbery.
        (b)  The actor's belief under Subsection (a)(2) that the 
 deadly force was immediately necessary as described by that 
 subdivision is presumed to be reasonable if the actor knew or had 
 reason to believe that the person against whom the deadly force was 
 used:
              (1)  unlawfully entered, or was attempting to enter 
 unlawfully, the actor's habitation, vehicle, or place of business 
 or employment;
              (2)  unlawfully removed, or was attempting to remove 
 unlawfully, the actor from the actor's habitation, vehicle, or 
 place of business or employment of the actor; or
              (3)  was committing or attempting to commit an offense 
 described by Subsection (a)(2)(B) [The requirement imposed by 
 Subsection (a)(2) does not apply to an actor who uses force against 
 a person who is at the time of the use of force committing an offense 
 of unlawful entry in the habitation of the actor].
        (c)  A person who has a right to be present at the location 
 where the deadly force is used, who has not provoked the person 
 against whom the deadly force is used, and who is not engaged in 
 criminal activity at the time the deadly force is used is not 
 required to retreat before using deadly force as described by this 
 section.
        (d)  For purposes of Subsection (a)(2), in determining 
 whether an actor described by Subsection (c) reasonably believed 
 that the use of deadly force was necessary, a finder of fact may not 
 consider whether the actor failed to retreat.
        SECTION 4.  Section 83.001, Civil Practice and Remedies 
 Code, is amended to read as follows:
        Sec. 83.001.  AFFIRMATIVE DEFENSE.  It is an affirmative 
 defense to a civil action for damages for personal injury or death 
 that the defendant, at the time the cause of action arose, was 
 justified in using force or deadly force under Subchapter C, 
 Chapter 9 [Section 9.32], Penal Code[, against a person who at the 
 time of the use of force was committing an offense of unlawful entry 
 in the habitation of the defendant].
        SECTION 5.  Chapter 83, Civil Practice and Remedies Code, is 
 amended by adding Section 83.002 to read as follows:
        Sec. 83.002.  COURT COSTS, ATTORNEY'S FEES, AND OTHER 
 EXPENSES.  A defendant who prevails in asserting the affirmative 
 defense described by Section 83.001 may recover from the plaintiff 
 all court costs, reasonable attorney's fees, earned income that was 
 lost as a result of the suit, and other reasonable expenses.
        SECTION 6.  (a)  Sections 9.31 and 9.32, Penal Code, as 
 amended by this Act, apply only to an offense committed on or after 
 the effective date of this Act.  An offense committed before the 
 effective date of this Act is covered by the law in effect when the 
 offense was committed, and the former law is continued in effect for 
 this purpose.  For the purposes of this subsection, an offense is 
 committed before the effective date of this Act if any element of 
 the offense occurs before the effective date.
        (b)  Section 83.001, Civil Practice and Remedies Code, as 
 amended by this Act, and Section 83.002, Civil Practice and 
 Remedies Code, as added by this Act, apply only to a cause of action 
 that accrues on or after the effective date of this Act.  An action 
 that accrued before the effective date of this Act is governed by 
 the law in effect at the time the action accrued, and that law is 
 continued in effect for that purpose.
        SECTION 7.  This Act takes effect September 1, 2007.