The morality of killing the baby is complicated by the situation. If the Order fails, the world gets destroyed, and the dwarves, Kudzu included, all get sent to a bad afterlife. If Belkar's faced with a hypothetical situation where that will happen unless he kills Kudzu, logically the moral thing would be to kill him, because Kudzu dying in battle would be doing Kudzu (and therefore Durkon) a favor. People aren't purely logical, though, and most of us would struggle to do such a thing. Belkar has the stomach to make that sort of hard choice, though, so if the Order needed to kill a baby for altruistic reasons he'd be the one to turn to.

All of that said, Belkar probably wouldn't be thinking through the moral dilemma under the circumstances and would probably act out of emotion. My instinct is that means he wouldn't kill Kudzu, but others might disagree.