I'm thinking about trying to date my friend's ex.
Why would you want to date your friends ex?
I'm really jealous of him and I feel like it would send him a message that he isn't better than me.
Wanting to feel better about yourself is fine but you shouldn't date your friends ex. It's really rude to your friend to not at least try to get the okay from him. You're also ding it for petty reasons so the relationship is bound to fail.
I don't care. Someone has to be his ex.
It seems very petty to do this. You should explore other alternatives because you could ruin your friendship.