It'd be like having a jury pool made up of serial killers.
These kinds of bottom of the barrel places of employment tend to have their policies and procedures tuned to make sure employees do things the way the company wants them done. That's just the nature of doing an unskilled, non-laborious job for a BigCo. There will be automatic spot checks, audits, reviews, etc. It is no different than the cube farm on the next floor where they review images of documents that AI systems have flagged as possibly fraudulent. Such systems are likely in place already for their current moderators.
Yes, but their hiring pool tends to be "everyone".
Those procedures might not work as well if your hiring pool is explicitly confined to "the people who watch ISIS torture videos for fun". I suspect standard policies and procedures include "fire the people who get off on this stuff".
They have little power to abuse and obvious accountability. If they block kitten and baby pics and allow people being tortured to death they'll get fired quickly after the inevitable shitstorm.
Insane and unhealthy or creepy doesn't neccessarily mean dangerous. Even if they got twisted pleasure from it they would be gross but not inflicting actual harm. It isn't like people are being brutally murdered to give them kicks - they aren't a source of demand.