It can easily be Fatah ...completely on their own. Fatah/ PA have been on the outs with one another since Hamas became a political power. They hate one another and figh each other behind the scenes. They cannot even come to some kind of power sharing agreement. As far as I am concerned if the Mossad was involved, the involvement was reversed. Information only. The actual lkilling was probably done by the Palestinians.
Key U.S. House Democrat cites troublesome issues in Iran nuclear deal (Reuters)
from the article: Hamas: Assisting Zionist enemy will result in confrontation