Re these "suspected collaborators" Hamas kills in droves

Seems to be a popular pastime Hamas encourages in Gaza strip each time something goes pear-shaped there:

Hamas militants killed seven Palestinians suspected of collaborating with Israel in a public execution in a central Gaza square on Friday, witnesses and a Hamas website said.
...
Another 11 people suspected of collaborating with Israel were killed by gunmen at an abandoned police station in Gaza earlier on Friday, Hamas security officials said.

I wonder: do all these folks volunteer for the duty? Or is there a competition for the role?