I really don't like my flatmate.
What did he do to make you not like him?
I've asked him to do things for me several times and till now I haven't heard back from him.
You won't always like the people you live with, but they should at least pull their weight with household chores. You should talk to them in person about what you think is fair for them to do.
Nah, I'm going to make his life a living hell, he deserves it!
If they cannot pull their weight reasonably you should see if you can find somewhere else to live. You should not have to put up with a slob.