A "criminal brain" did NOT make the creation a "monster"
As ominous as Fritz stealing a "criminal brain" might be, the film itself doesn't use that as the basis of the monster's destructiveness. All of the creature's scary actions are either accidental or self-defence. Most notably, he didn't mean to drown the little girl, he wanted to play with her. He killed Fritz because Fritz was tormenting him; he killed the Professor because the Professor was going to kill him. Arguably, the only significance the "criminal brain" has on the plot was that it was why the Professor wanted to kill the creature.
In a way, it's vindicating to see a "criminal brain" doesn't make the monster evil, because really, the whole idea is nonsense. It's phrenology, a pseudoscience. A brain doesn't necessarily make you evil- nurture has more say in that.