The other advice here might be good but might not be practical for your situation. No idea. I don’t envy you though.
This isn’t something where strangers can’t give you a correct answer because we cannot know all the complexities of the situation and what is up with this kid.
But why not let this play out?
He tries to do his murder plan. Insta-kill doesn’t really work in 5e unless you want to. So one PC takes damage (potentially), wakes up… all PCs now awake and he is standing with a bloody knife over his party member. Equally it can be as simple as a session 0 (or now) rule saying no PC on PC fighting (for all the social reasons and also because dnd’s rules are not balanced for it).
Actions have consequences. His character would not survive and/or stay with the party… lesson learned?
The party is also going to be more suspicious and less trusting of sketchy behaviour with a new member after that so his new character would probably need to be a little better behaved etc etc.
Murder hobo game play gets murder hobo consequences. Have fun role playing your character being put in jail and sentenced to death (assuming they don’t fight the guards and die immediately) while the rest of the party goes and has an adventure.
But that’s just my approach. Lean into it, show (don’t tell) how being a **** to those around you doesn’t work out well.