User blog comment:TheWarrior29/Who killed the Watcher?/@comment-11297966-20140129004829

I think it could be Thane because Thanos in Nova Vol 5 8 was telling his Black Order how he is a huge threat and the Ebony Maw might agree and use Thane to kill him. It could also be the Unseen from Point One Vol 1 1 or whoever was hiring him.