You can not kill Sinister because there isn't just one Sinister. The real Mr Sinister is dead this is a clone with mutant DNA. He has many clones of himself hidden everywhere. It's no different than Dr Doom with his Doombots.
Sinister has so many clone factories laying around just look at Hellions book. You guys keep trying to theory craft a way for Sinister to not be around but it is silly.
No one trust Sinister no one likes Sinister no one is Sinister's friend. Heck even while on the island he just keeping Psylocke's child away just to use her. It was a deal with the devil showing the lengths that mutants are willing to go to for mutants. No body knows DNA like him and they know it.
All of your guys story ideas are just a waste of paper for a simple reason of "ooo Sinister bad" like no body knows that better than the mutants.