I don't think that most feminists hate men. Most people who identify as feminists are pretty liberal, Hillary-Clinton-esque feminists, or "fun"-feminists of the "Pole dancing is empowering!" sort.
Of the feminists that do hate men, I think most of them do because they have experienced oppression at the hands of men in very direct ways, such as:
* Sexual harassment (probably all of the women you know have been sexually harassed)
* Sexual assault (one in four on average, higher than that for women in the global majority/women of color)
* Overt sexism (being derided as an "affirmative action" placement, told to make sandwiches/get off the Internet, etc.)
* Covert sexism (being assumed to be a subordinate in an office, being assumed to be an assistant/boyfriend at tech conferences, being singled out for "preferential" treatment in a Victorian expression of respect for fragility)
As a man, I think that there are a lot of reasons why women would hate men, and why people of color would hate whites. I think this is mostly justified. Men, and particularly white men, have and continue to do pretty absurdly horrible things, and more than that, refuse to even admit to it! The catcaller on a street doesn't think he's doing something wrong, much less the person who judges a woman more harshly for her efficacy at work (since she seems "cold" if she gets things done).
One of the best defenses of hate I've ever seen is from Butch Slaughter. You can see some of his work here: https://www.youtube.com/results?search_query=Butch%20Slaught...
Mr. Slaughter guest lectured at a class I took on domestic violence, and one of the things he said that will always stick with me is that "there are some things you're supposed to hate." Systemic inequality, and the oppression of the majority of humans, is definitely something you're supposed to hate. And I think it's impossible to wholly separate that from the people who are responsible, either by accident or by choice, in supporting such things.