Ok, so as far as i know no one has posted this yet... i know there are a lot of other "instant zombie" threads going around, but this is somewhat different.
What you do.
1. Be a warlock (locks ftw)
2. Cast Life tap until your life is all the way down
3. Use Hellfire and die.
4. Have fun as a zombie!
I know someone has obviously posted that you can kill yourself as a warlock before... but no one seems to have connected 2 and 2 yet (22).
Alternatively, you could just hellfire your life down.