The casual way they were conducting themselves reveals it.
This wasn't a one time event, this was business as usual and they weren't worried about getting caught at all.
If he hadn't died, this would have just been "injured while resisting" or some such and nobody would have ever even looked at the video or believed a thing the inmate said.
Of course this is not the first time. Will not be the last time either.
These places are not country clubs. Not nice places at all.