I don't think that you and kilobug are actually in contradiction. Kilobug is saying that the deaths of the Death Eaters is a negative term in the utility function. You are saying that the total utility of the act that kills them is positive (or greater than the alternatives). Unfortunately, idiomatic English discusses these two very different points in similar language.

In other words: kilobug says that, as you survey the consequences of Harry's act of transfiguration, when you get to these deaths, you do not rejoice; buybuydandavis says that, considering all of the consequences of Harry's act of transfiguration, you rejoice. At least, that's how I understand you two.