In my mind, anything with agency and power can and will start to think for itself. Reaper have been doing nothing *but* reaping for time out of mind.

Suddenly, people are dead and alive and dead and alive, the Apocalypse is on/off/on/off. The Devil's offering deals, and the angel's are, too....

What *shouldn't* Reaper's go rogue and try to take advantage like every other being is?

In the Layla ep, Sam said that binding the Reaper was like 'putting a leash on a great white' - so it seems more than likely to me that they'd break out, here and there, and do their own thing.