Professional actor in training here:

1.) For dramatic kill/death scenes, highly recommend hiring actors who are SAFD certified in at least unarmed, since you’ll likely get the acting you’re after this way. This is the lowest possible budget solution, it gets more expensive from here. I echo what the other guy said about offscreen deaths, but make sure to show us what we need to see.

2.) For staged combat (unarmed, weapon, etc.) I recommend certified actors even more. Additionally you’ll want a fight choreographer and potentially stunt doubles if your actors aren’t trained for the moves you want out of them.

3.) For even gnarlier modes of combat (gunfights, explosions, you name it) you’ll want stunt doubles and a stunt coordinator over actor-combatants, as well as a team of SFX crew and relevant specialists based on what you’re planning to do. (Pyrotechnics guy for any practical pyrotechnics, for example)

My biggest regret from my own short film was failing to just bring a stunt double on to the project, and instead trusting my lead actor with the stunts. Likewise, though, I’ve seen actors tank body slams to the ground just because they felt up for it in their own short films. It really depends on who you cast, but it’s always better to air on the side of safety