I believe most monsters were okay with the previous child-murders because one or several of these reasons:
A: They would consider them a different species/race separated from themselves and thus it doesn't count as murder.
B: They're angry with humans because their royal heir died by their hands and their enforced isolation (note that by the time the Fallen Child came in, the barrier was already placed long ago).
C: They're bitter with the War, especially considering the onslaught that followed.
D: Their minds are partially alien and don't understand "standard" morality (they're called monsters for a reason).
E: All of the above.
That naturally changed when the protagonist came in. Amusingly enough, their king's reasoning of using the human souls to become almighty and exterminate humanity was
precisely what humans feared that one day would happen, hence why they started the War. I guess that's the so-called irony of fate. Or perhaps idiocy and lack of foresight are universal.
.
On the other hand, the majority of the monsters do not present a physical threat to the average human kid and are extremely brittle, so perhaps going back to the surface and joining humanity was not a good decision. But it's the thought what counts.
Moreover, the Amalgates
are twisted undead abominations that do try to kill you, just because you were unlucky enough to be in the way and/or because they were hungry. But all is forgiven because no-one died by their actions
and the main character i
s human scum who doesn't count.