I would argue that, while "evil" cannot be defined in precise terms, you can definitely say that evil acts are unnatural acts that only humans are capable of. Why? Well for one because a natural foodchain is balanced and biased towards long term survival of all the species it involves and towards further creation of life. It's a little ironic that human beings are the only ones in the entire animal kingdom that can choose how to live, we are the only ones able to choose what are, our intellect transcending our DNA coding, yet we are also the only ones destroying our habitat and each other. Isn't that funny?
But back to Hitler, if you're trying to make the case that his evilness is subjective, I mean no offense but that's a really dumb argument. Hitler was in no small part evil because his actions were, on one hand irrational, fueling and amplifying his people's potential for hatred and destruction and on the other hand detrimental to the survival of our species and of Earth itself. And again, genocide is not natural. You see, in nature animals kill to eat, but that's to satisfy a basic necessity and not out of some wicked sense of justice and animals can definitely not kill on an industrial scale like we do. Whatever definition for "evil" you find, genocide on an industrial scale is pure evil by definition.
And if that doesn't sound objective enough, consider that culture is a part of who we are. We aren't DNA-coded to eat certain foods, or to live in a certain place, or in a certain way. Compared with rats, we can rely on the wisdom of our elders in order to survive. And we've survived this way for a long time. As an example, our rich culture, which includes preconceptions and taboos, is what prevents us to eat each other, or to have sex with our siblings, or to bring human sacrifices to our gods. Actually some preconceptions are more subtle and newer than others - for example the notion that children are fragile beings that need to be loved and protected, instead of someone's property, is pretty new, being popularized by Christianity.
So you know, if popular conception is that doing this or that is evil or toxic or taboo, there's a high probability that such judgments are correct, helping us to survive and thrive. Even with all the false positives (which tends to be the lack of tolerance towards people that are different from us), dismissing our heritage would not be wise. Plus usually the guidelines are simple, like being a murdering maniac counts as evil, though somebody should tell those jihadists.