It's not semantics.

If you killed someone committing their own separate crime, you'd be both a killer and potentially a hero.

It is important to make clear he both committed a very serious crime and potentially blew the whistle in act of bravery.

Not if they were breaking into your house and it was self-defense. Then you're technically a killer, having killed someone, but you're more a hero, and not criminally culpable at all.