He destroyed an evil threat by using an expedient manner. He didn't torture them. He didn't kill them after they surrendered. He didn't chase them and terrorize them with taunts before killing them.The act of killing isn't necessarily evil; the motives and means play heavily into it. In this case the means was morally neutral, while the goal was good.
Keep in mind that in
4e there are no game mechanics to penalize actions that are out of alignment bounds. The only means are social. A Paladin of Pelor who goes on a murderous serial killing rampage retains all of his powers but would likely be hunted down and killed by his fellow Paladins, and would find no solace in Pelor's churches.
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