Hatred in general is evil: because most people are good, and it is wrong to assume the worst about them unless given a very good reason to do so.
However, it *is* proper to hate things like Hitler, slavery, racism, etc. In other words, it is proper to hate people and ideas that harm, dehumanize, and ultimately kill other human beings.
What gets complicated, however, is considering the origins of such hate. Given that most people are fundamentally good and not primarily motivated by hate, what made those same people misguidedly believe in horrible things like Hitler, slavery, etc.?
I think: Out of a misplaced sense of love.
Love for a national leader. Love for one's own race. Love for tradition. Love for material wealth.
A little bit of love for each of those things is not necessarily bad. The problems arise when they are carried to an extreme.
When we allow our leaders to amass so much power that they turn into Hitler. When we love our race so much that we forget that those of other races are human beings, too. When our love for tradition becomes a stubborn unwillingness to break the mold, because "that's just how we've always done things." When our love for material wealth tramples our concern for other human beings.
There is no easy answer here, except the application of rationality and common-sense to figure out what is proper to hate and what is proper to love, and how much to love and how much to hate those things.