Could just send them for 15 years in a very harsh prison if they say otherwise, if that's what they already do for anti-war speak anyway?
So, mistreating veterans might not work out as expected.
More details: https://en.wikipedia.org/wiki/Wagner_Group_rebellion
The deal seems to not work out.
I think there were no official explanation for this incident, propaganda posts explain that they were using drugs and playing with a grenade onboard.