[Slings and Arrows]

In Tom Clancy's Patriot Games, the book mentions in passing that the psycho main-antagonist terrorist was quite good at his job as a counterintelligence officer: Almost half the people he killed for being informers actually were informers. I doubt that Hamas even rises to that level.

[dennisw]

And half the reason Hamas is wacking Palestinians that it merely suspects of spying is to strike fear in the hearts of Palestinians who are thinking about spying. To deter them from doing so. Ye olde heads on pikes approach

Israel pays Palestinian spies. This is the main incentive