how bad really was hitler?

Really depends how racist you are and how you feel about killing massive amounts of human beings ( and ofc exploiting them too before that ). My opinion tho? Well, kinda bad ( we have to thank him for the atomic bomb after all, his war created it ), but without his doctor's experiments, we wouldn't have the modern genetics as developed as we have them, so I'll leave it up to you to come up with your own conclusion out of this.