It's the easiest, most efficient way to achieve his goals.
Now we're going in circles, let's just agree to disagree and call it a day.
Halsin is cool. He seems more morally centered than any of our actual companions. In these circumstances, killing goblin children is not an evil act. Typically, we would see killing helpless noncombatants as evil. And in our world, children are usually helpless noncombatants. But these goblin children are not. They are dangerous, vicious, and either have already murdered, or will almost certainly do so soon.
Agree 100% He might be slightly reckless leaving the grove amidst it's refugee crisis and chasing off with random adventurers, but judging by his correspondence in the library he's tried to get outside help and is genuinely worried about them tadpoles (with a good reason too)