Runeterra, or at least the parts of Runeterra we know, has an ongoing problem with locking people away only for things to go horribly wrong. In review:
-Syndra did bad magic, the Spirit of Ionia locked her away, she got out and was justifiably enraged. Now she wants to do massive damage.
-The Darkin pretty much ruined whatever parts of the planet they "ruled," the Aspects locked some of them away, they got out. With their minds warped if not outright broken by centuries of solitary confinement and sensory deprivation, they're even more destructive than before.
-Thresh was assigned to guard a vault of evil relics, the relics (shocker!) turned him evil, he got out, and now he's just torturing people willy-nilly. In this case the issue is either that Thresh was unsupervised or that so many dark relics were put in the same spot in the first place with no one knowing that they could apparently turn people evil.
-Xerath murdered Azir and stole Ascension, he got locked in a sarcophagus and then a tomb, got out, and now wants to take over the world. He's a bit of an exception because there really didn't seem to be any other way to stop him, but he still qualifies for the list.
-Urgot would likely have opposed Swain's revolution, so he got sent to the Drudge, got out (noticing a pattern here?) and now wants to bring violent anarchy to the world so that the strong can rise from the ashes. Sure sucks if you're not one of them though.
-And now we have Sylas, who killed people with magic as a CHILD (largely by accident, too), got locked away, got out, and now wants to wreck the Demacian government through the most violent means available.
Now I'm not saying the solution is "kill all the threats" because that'd lead to a NEW list of problems, but we clearly need some serious prison reform, magical or otherwise.