From what I understood Hitler ate as a vegetarian because he felt that his first and foremost obligation was to be the best he could for the German people and that included being as healthy as he could for the German people.

That was also the reason from what I understand that he didnt have a wife (until the very end) and didnt have children. He didnt want his time split that way.

I dont think he was a vegetarian for any type of humane purposes at all. I think he also stated somewhere that he likes living off the land.