As has been previously stated; (for the duration please assume we're talking about a state without civil/criminal immunity)
Any time you use or sometimes even threaten to use a weapon criminal charges can be filed. Civil complaints for a number of things. Depending on whether of not the "victim" of your gunshot lives you could face things like: Wrongful death, Lost wages, pain and suffering, undue mental distress.
Civil cases are based not like our criminal system which uses the standard of "beyond reasonable doubt", rather the civil system uses "the preponderance of the evidence" or in other words which lawyer can manipulate the triers of the facts better.
Be prepared to lose every penny you have, or will have, and be prepared to be in court for an extended period of time. Even if it were the most righteous shoot of history. All of this sucks, but being dead sucks more.
As Massad Ayoob puts it "You have more power than any politician, president, lawman, or judge to take the life of another based soley on your own summary, review and command, without the possibility of appeal for the person you have essentially sentenced to death. it is a responsibility that must be taken extremely serious.