Personally, Angor wouldn't mind rescuing the drowning kid.
The panicking villagers confirmed it for him, that Kumur went into the water all by himself just to spook the other kids. Kumur intentionally stayed in the water for longer. And when he decided to return, he could not.
In Angor's view, letting the rowdy kid stay there and face the monster as a live bait seemed like a good lesson. This was why he did not move to intervene yet. He was confident that Toby could easily reach there and snatch Kumur back before the monster could do anything.
Angor was planning to wait until the last second to get Kumur out so that the naughty kid would learn how to behave in his life. But… he wasn't expecting Tulu to interrupt his plan.
And he didn't like it.
While it was true that there was no absolute righteousness or injustice in the wizarding world, Angor usually preferred to act as a "lawful" wizard, or white wizard, because of Jon's education.