Rarely have I seen a more irresponsible view of such a thing.
It's not my personal view of the situation, it's just me trying to view things from the elves' point of view. I agree that the bastard should be strung up, but that's not the point of the discussion.
The seal on this Jerkqu's powers also seems extremely flimsy. He could still control and summon a whole army of monsters.
He is able to control and summon an army of monsters with the aid of a staff. A staff, mind you, that he most likely received from a human. Sure, maybe he stole it, or killed someone for it, but given the fact that he's working for humans, they most likely gave it to him to make him a more valuable asset. That being said, the last line implies that the sealing will be removed once his arms are gone, so, yeah, the elves could have done a much better job at crippling him.
It's the elves' problem if they have such stiff laws. Surely they then have prisons to keep the dangerous inviduals locked away. Oh, right, they don't, they just kick out the murderers to be the human countries' problem.
If the elves here are your typical long-lived, slow to reproduce sort, they most likely have very few criminals at all. If this is the case, this guy here might be the first real case of a monster coming from their society. A peaceful society has difficulty executing heinous criminals, even when it is clearly the best course of action. Keeping him locked up is definitely a good solution, but considering the long lives of elves (theoretically), that might not be feasible. Crippling him and exiling him to certain death is as good an option as any. That said, they missed the mark by making it so easy to get around the seals. I would guess that they did not expect the humans to so readily help a bloodthirsty murderer.
I'm sure the elves are also pleased when humans weaken themselves by killing each other. Two birds with a single stone for the elves.
You are inferring
a lot about the elves here. You are giving them a Machiavellian view of the world, where they view humans as an opposing force that must be contested and controlled, whereas they could very easily just be isolationists who did not expect their actions to have a great impact. Not all societies act with malice, and this entire situation is most likely caused my misjudgment of character, on both the elves' and humans' part.
To be fair, I will freely admit that I am giving the elves a lot of credit here. In the end, we can't really make any judgement here because we know next to nothing about them. I don't know about you, but given the genre we're dealing with here, I do not expect a good deal of quality writing, nor do I expect a satisfying explanation of the elves' morality.