The Keep I watched last weekend with Batgirl.
The question is, why? Why didn't those Ancient Powers of Good who managed to imprison the Evil Thing simply not destroy it?
In my novel Sword of the Gods, and its sequel Spinner of Lies, this is something I've had to wrestle with.
The following list are "reasons to imprison, not kill" that I've come up with, along with those I've boiled down from the great feedback generated in comments where I first posted this:
• Death would release the slain Evil Thing (or the slain Evil Thing's essence) to an inevitable reincarnation at an unknown time and place;
• Moral reluctance on the part of the victors;
• Inability for the Evil Thing to actually die (for whatever reason);
• The Evil Thing's potential as a weapon in some other conflict the victors are concerned about;
• The Evil Thing is linked to the Powers of Good so that to slay one slays both;
• Death would be too easy for the Evil Thing--eternal punishment is what it's jailors wish it to receive;
• And finally, some Evil Things are slain (we just don't usually hear about those in stories).
(Of course, if you're not an ancient power of good, but just a really mortal with weapons forged for a god, it's more viscerally satisfying to kill an Evil Thing than imprison it. A mortal's short life span means that killing a powerful demonic entity seems to get the job done well enough, so why worry about future generations who might have to deal with "evil essence" reformed somewhere else?)