A good guy killed a bad guy.
Why can't we just leave it at that?
Because it isn't ever that simple.
If the shoot wasn't legal, this it is not a matter of a good guy shooting a bad guy, but a bad guy shooting a bad guy.
If the shoot was legal, good for the homeowner.
Some are arguing that this shoot was at worst illegal and at best immoral and that regardless of the legalities, the homeowner was wrong.
Personally, other people's morals are for other people. They often like to inflict them on others in claiming moral high ground as justification and throwing out insults or mockery as a way of trying to bolster the appearance of the position being right, but such tactics don't actually make the position any more right and are actually rhetoric/argument fallacy of ad hominem.
The position being taken is that if you take the perspective of the homeowner being in the right, you are a chest thumper - which is in the connotation of being a negative attribute. You are a chest thumper not because the home owner's decision to act was necessarily illegal, but because certain people feel he acted wrongly as per their standards and hence anyone who doesn't agree gets the negative entitlement.