The poor, helpless innocent is imprisoned by the government, or by the bad guy. So of course, our helpful hero decides to release them!
Only... Nice Job Breaking It, Hero
. That "innocent" being imprisoned was actually a violent, powerful threat, and 'you' just realeased it on the world. The people imprisoning it may be jerks, but they were right to keep it imprisoned.
It doesn't have to be literally imprisoned, it can be restricted from movement in some other way. Whether the prisoner themselves knows that they are dangerous can be up for grabs, so long as they seem innocent and really, really aren't.
- Spacetrawler: the Eebs are enslaved by the other intelligent races, but in the process of freeing them the heroes find out that they are violent and try to kill everyone.
- Lil' Mell: Homeschool Joe isn't allowed to go anywhere off his Kid-leash(tm). Turns out there was a reason for that...