My son saying bad stuff...?

My son is 3 1/2 yrs old and he's been saying some awful things. He's been taking his Buzz Lightyear or Optimus Prime toy, pointing the laser at me or my boyfriend and say,"I'm going to kill you." He's also says other very hurtful words. I've been telling him that we need to be good people and to not say things like that because it's not nice, but he never listens. I'm at a loss on what to do about this and I'm very concerned. Does anyone have any ideas?