Why do men seem to be more heartless than women and why do women let them get away with it...?

It's not all that simple, everybody is sufferer be and man or woman, just as many evil of both. The reason you've heard of more evil men I'd quite simply men have been the ruling over the years, no offense if course, but they have been dominant with more prominent figures. As for the other thing with the cheating and what not, women are more emotional and susceptible to the whole "but I love him" deal, and men are more ran by logic than emotions.