I'm going to hook up with my friend's ex. I want to just hook up with him for a bit then end it an apologize to my friend.
Why would you want to hook up with him the first place then?
I've always wanted to see how good someone my friend used to date is in bed.
Being curious is fine but you're doing this is a very backhanded way. If you want to hook up with her ex you should just tell her first and see if she's okay with it so you don't risk damaging your friendship. This is particularly true if it there's any risk of this becoming more than a one night stand.
Noone says I have to tell her. Fuck her she always gets all the good guys. She deserves to suffer a little.
I think that maybe the bigger issue here is you don't care about your friends feelings?  What kind of friend claims that a friend of theirs deserves to suffer?  Why would you say that?