* Intent to kill or harm someone - murder (with degree being dependent on how much pre-meditation you had)

* Intent to pull the trigger, but not to kill or hurt anyone - manslaughter

* No intent to pull the trigger (eg accidental discharge - maybe the trigger pull on the gun is very light and you put a little too much pressure on it) - possibly manslaughter on recklessness grounds if you knew there were people around or the person was there, but also maybe a lower crime (or not a crime at all) if you were being careful with the gun and got unlucky about where it was pointing when it discharged

* Not mentally competent to understand that you might be hurting someone or that hurting someone is wrong - not guilty because of insanity

* Intent to kill the person in self-defense and the person is intending to kill or harm you - not guilty of any crime due to self defense (a murder, but not a crime)

* Intent to kill someone else who is attacking you in self-defense, but you missed - also self defense (intent follows the bullet)

There are probably 10+ more other major scenarios to cover here with different outcomes. This is partly why when someone kills someone else with a gun, the prosecution tries to hit them with murder, manslaughter, and reckless handling of a deadly weapon. They don't know which one will stick, but they can be pretty sure that a jury will find one kind of intent.

you can't have direct evidence of someones alertness or state of mind, only circumstantial.

the direct evidence from witness testimony is only needed if the shooting is disputed.

assuming the shooting is proven, with no evidence of intent, you can't have murder 1, and likely can't have murder 2 either and are probably stuck with involuntary manslaughter.

obviously it depends on the state.

however even manslaughter requires some requisite knowledge and intention of what you are doing - but proving something like insanity or incapacity is usually on the defense to prove.

In this example, it's probably open-and-shut 2nd degree but the prosecution now has to prove that you premeditated this if they want to go for 1st degree (IANAL). They don't get to just say "it was clearly premeditated" and then put the onus of proof on the defendant.