Hitler doesn’t need to be insane to be a terrible person. Why make things worse than they are?

We may be working with different definitions of "insane", here. We don't know exactly what went on, it's kinda hard to diagnose the dead, but symptom-wise his well-attested levels of mania as well as depression were anything but the sign of a well-adjusted personality. And taking meth to deal with the depression part certainly didn't help. Could be due to multiple underlying factors, though, borderline, narcissism, plain old PTSD, a nasty neurosis, all are candidates.

Hitler’s three critical missteps

Strategically, possibly, as in those things being the straws that broke the war machine's back. Doctrinally everything about the war was straight nuts, though, like the no retreat orders. Not to mention starting it in the first place.