The distinction from the bandits to me was that the bandit women were grown adults that could be held fully accountable for their banditry. This young troll is for the moment at least still just a grieving child literally crying for its mother. If it comes back later as a murderer then I'll kill it without hesitation. But at the moment it's just a child and one of a sapient species capable of language/ communication. If it was an entirely feral beast incapable of any speech even with magic then that's also a factor that could have tipped me over toward putting it down.
As natural born human eaters, trolls are inimical to human life.
Even if you find an individual troll who is an exception to the rule. A vegetarian troll so to speak...
Some species just come into conflict like that.
rather than morality, it is a matter of interest of the species. it is in the interest of humanity to exterminate natural born human eaters
and it is in their interest to kill and eat humans
sometimes, such creatures can form an organized society.
and it becomes plausible to negotiate a "X won't eat humans, and humans won't try to exterminate X"
X will live in their own walled off nations. you could potentially coexist with such creatures as distant neighbors. (although it is arguable if you even should instead of war with each other)
But trolls do not seem to be like that.
trolls do not seem to have a civilization you can negotiate with. they just wander around eating anything any living creature they find, including humans.