I don't think they're unrealistic per se, but in the context of the Ultimate Universe, I think it makes more sense to have mean villains, especially in USM.
In ASM, most of Spidey's powered villains were born out of accident. In USM, they were a part of that big race to get super-powered beings, and since most of those experiments were banned, they had to use people that nobody would regret, and preferably people that could be used as soldiers or bodyguards. Hence Electro and Sandman. We saw what happens when a civilian with no training ends up with powers : the Shocker.
Magneto is a new Hitler, and was portrayed as such as far as in the Return of the King story arc (actually, Ultimatum just tells us the story of what would have happened if Magneto had succeed in Return of the King). He is sympathetic of other mutants, even when they oppose him, because they are part of "his" people. If he was to be the greatest threat in the Ultimate Universe, he had to be powerful and evil.