I'm thinking of cancelling my plans.
Do you have a reason for canceling them?
I'm going to tell him that I've planned my death and I'm never coming back.
You shouldn't cancel plans at the last minute since it will make people less likely to trust you in the future. Especially since with your excuse they will know you are lying. Canceling plans with a blatant lie will just make people angry at you and not want to be friends anymore.
Why should I keep my plans with this loser?
Faking your death is a federal offense. Why not just tell this person that you don't want to hang out anymore and end the relationship. It will make you look better and you won't be considered insane.