Seriously killing him would be easier even if surviving the whole ordeal us low if they do succeed they now have a very powerful mage who is very angry
It's like The Mummy, where you placed the curse on the criminal so that he won't die, and somehow will return to end the world, instead of just execute him.
And that is why I always advocate on just killing the bad guy. Preferably while they're still in the middle of their first sentence. Giving them even a second more time is a risk. I wonder how many of the associations mages are going to be killed by him once he inevitably escapes. Hopefully he kills the black haired woman in the most horrible and painful way. Because that cunt is a dumb fuck and deserves to be killed by the villain, preferably after having been horribly violated so that her mind is completely broken. Not because it would teach them anything, but just because she's a dumbass.
Makes me think of a few new plausible scenarios regarding
PoH from SAO, after he was transmuted into a tree by Kirino, just before the world accelerated. Thing is in the webnovel he died, but in the light novel they never did find the body later 'cue ominous music'
You'd think an association of mages would realize that giving a hostile mage thousands of years for self-study completely safe from any outside interference is a bad idea.