There are 3 code sections that govern when lethal or deadly force may lawfully be used.
1. Defense from a forcible felony; A person is justified in using force which may harm or kill only if he or she reasonably believes that such force is necessary to prevent death or great bodily injury to himself or herself or a third person or to prevent the commission of a forcible felony. You are not justified if you were the aggressor or you are/were/on-the-way-to committing a felony. (The state has pre-empted local cities and counties from further restricting this defense.)(16-3-21)
2. Defense of habitation; (here habitation means dwelling, motor vehicle, or place of business) A person is justified in the use of force which is intended or likely to cause death or great bodily harm only if:
A person is breaking\has broken into your home in a violent and tumultuous manner, and you think that the intruder is going to assault you or someone else living there.
A person who is not a member of the family or household and who unlawfully and forcibly enters the residence and you know it is an unlawful entry.
The person using such force reasonably believes that the entry is made or attempted for the purpose of committing a felony therein and that such force is necessary to prevent the commission of the felony.
3. Defense of property other than habitation; Lethal force cannot be used to protect personal property unless the person using such force reasonably believes that it is necessary to prevent the commission of a forcible felony.(16-3-24)
(Stand Your Ground/Shoot First/License To Murder - went into effect July 1st, 2006) If you have determined you need to use lethal force (as stated in one of the underlined "Defense" sections immediatly above) you do not have to try to retreat before using that force. If your defense is valid, you are immune from criminal procecution (unless it is illegal to carry that weapon where you used it) and civil liability actions.(16-3-23.1, 16-3-24.2, 51-11-9)