I'm going to leave my flatmate before he leaves first and ditches me with the lease.
Why do you think your flatmate will ditch you with the lease?
I'm tired of listening to him complain about my roommate's bad behavior.
It can be frustrating to be stuck in a lease with someone who bothers you, but you can't just walk out on a lease. Unless there is a serious emergency, your roommates will rely on you to pay your fare share. Try to work out your problems with your roommate, running away won't solve anything.
We'll just leave each other alone and everything will be okay.
That is a good approach. You don't have to like each other but you should try and get along.