So yes, Hitler wasn't a traditional rightist, but he didn't quite fit in with the left. In some ways he's an asterisk figure who shows how the simple left-right spectrum doesn't account for everything that happens in politics. What's left and what's right changes over time and some politicians have combined left-wing and right-wing attitudes to produce some strange and terrible hybrids.
For the same reason the Bloods hate the Crips, rivals fighting over the same "turf".