Oh, please.
If I saw someone messing with my truck and he ran off, and then by some bizarre chance I saw him again an hour later with a police officer nearby and I said: "Officer! Arrest that man! He tried to break into my truck!" it would be purely my word against his and the would-be thief would walk away laughing.
I have never heard of someone returning to kill a witness who saw him trying to break into a car.
Even if your scenario was in any way plausible, you're still not allowed to kill someone because you believe that they might try to kill you tomorrow.
In some states you are.
Many states have a “perception” clause that says even if there’s no overt threat to your life, if you perceive one from someone, you’re justified in using deadly force.
Stuff you’ve never seen continues to shock and amaze others. Over in DC they had gang-bangers assassinating witnesses to little more than pickup truck breakins.
I’d suggest you not ever visit DC. They’ll have you for lunch.