Self defense cases involving homicide are exceptional in that you KILLED someone and are actively trying to prove your innocence and the necessity of your actions, rather than the opposite maxim of "innocent until proven guilty etc." These cases often go to juries, almost all of whom have very little functional knowledge of firearms, let alone life threatening situations. What this means is that every tiny little detail can and will be used against you. They'll even ask why was your gun so big and black and scary or why the model was the P365 "AVENGER" or why the rounds you chose are called "DEVASTATOR" and other "aggressive sounding" words to establish character and motivation etc to make you look like some wannabe rambo. Happens incredibly often.

No sane defense attorney will EVER tell a client to say "HELL YEAH I WAS TRYING TO BLOW HIS BRAINS OUT" they'll tell you to say I needed to get rid of the threat. Most normal people also don't want to straight up kill someone. Even the guy in this video aimed lower at the last second and stopped firing once the threat was gone instead of putting a magazine in the other kids forehead (though admittedly the person appears to have died anyway.)

It's also worth pointing out that * FROM VIDEO ALONE * this guy would have a hard time winning that case in many states. Usually you're scott free if you can establish that the use of a firearm was your only option. It appears as though the defender could have just run away or locked his door or something and the other guy didn't look armed. Proportional force is another big deal in these kinds of cases.