He's willing to kill Flint in order to take something that he has no right to take.
Hmh? The local points a gun at Kirk; Kirk tells him he's now fully within his rights to gun down the asshole and then throw the corpse in jail. Sounds like an everyday situation in any country where possessing a gun isn't illegal to begin with.
Kirk then adds that with Flint justly killed, Flint's possessions will also be free for taking, so it's a lose-lose situation for the stupid criminal. Flint backs off.
No "unnecessary killing". Merely a just threat of force for self-defense. By a de facto officer of the law, no less. Kirk really showed considerable restraint there.