On the issue or prior acts coming into evidence, here's what the rules are in general...
For the "victim" (i.e. the person who got shot), that person's prior history generally can't come into evidence. Why? Can't justify a shooting using information the shooter obtained after the shooting is all done. The shooting must be justified by the acts leading up to the shot. Now, someone in this crowd is going to say "but what if the shooter knew of the person's history of violence/prior threats/etc.?" That can change the rule somewhat, and you have an argument that can be made as long as you can demonstrate that the shooter had that knowledge before the incident. Example: Guy has repeatedly said "I'm going to cut your head off," and one day he's walking towards you with a machete in his hands. You can probably get those threats into evidence.
For the shooter, now it gets less clear. In general, a prosecutor cannot introduce "prior bad acts." In plain English, a prosecutor generally cannot introduce evidence of previous actions by the accused in an effort to show "he did it before, so he must have done it again." This rule (like almost all court rules) has exceptions, and among those are the use of prior acts to show motive, or to demonstrate a continuing course of conduct. The biggest is for use as impeachment, and that's where a good lawyer comes in. If the shooter is testifying and says something along the lines of "I would never pull my gun on someone because of (X reason)," the prosecutor can then introduce evidence of all the times you did pull your gun for that reason.
Bottom line: assume that the prior history of the person shot isn't going to see the light of day at trial. And, live your life as if everything you do will be offered as evidence against you. Don't go around making racist statements. Don't go around threatening people. Don't go around saying you'll shot someone. Don't wear shirts that say "kill them all and let God sort them out." Don't create a history of drawing your weapon. Avoid giving evidence to the other side if, God forbid, you end up in a shooting and are on the legal defense.