2sense: No. The discussion of the “necessity” of evil is not fallacious. But I think it’s a pointless question (which is why I’ve taken so long to answer your question).
Should hurricanes, tornadoes, earthquakes exist? Should the laws of physics be what they are? Why should there be entropy?
“Should” is inane in these cases, as well as evil. Things either exists or they don’t exist. Evil (according to some definitions) does exist. Arguing whether it should exist is attempting to apply ethical philosophy to a natural phenomenon.
If pigs had wings, they’d be chickens.