Law Abiding Citizen
in case any anon has not seen it.
https://vimeo.com/463221008
Yes. I couldn't quite figure that out.
But if he was this genius C!A tactician, then maybe he knew details about his cell mate and why he was there. Maybe his cellmate was a murderer and he thought his vigilante justice was justified in killing him too. Seems like he could have figured out a different way to get into solitary confinement though.
But it's just a Hollyweird movie after all.