Why would believing ethnic cleansing is justified make someone deranged? I don't see a moral difference between saying "It's OK to kill them because they're young" and "It's OK to kill them because they're Chinese/Poles/Jews/Black/Plutonian".
First off, allow me to correct that I was referring to infanticide in cases of severe development issues. Second, a will to ethnically cleanse does not need derangement, it needs a true belief that one's own ethnic, political or religious stance justifies genocide.