PROFESSOR: And here, the abnormal brain of the typical criminal. Observe, ladies and gentlemen, the scarcity of convolutions on the frontal lobe as compared to that of the normal brain, and the distinct degeneration of the middle frontal lobe. All of these degenerate characteristics check amazingly with the history of the dead man before us, whose life was one of brutality, of violence and murder.
The professor is giving a little lesson on eugenics. Eugenics was the belief that some folks had good awesome genes which made them good and awesome, and other people were degenerate and less awesome. So the professor here is saying that criminals are criminals because they have degenerate brains.
Or, to put it another way, criminals are criminals because they're less than human. Eugenics used to be popular in the U.S.—and Hitler picked up on it as an excuse to kill Jews and black people and everyone else who he considered to be eugenically inferior to the German people. So, in the long run eugenics did much worse things than that poor criminal brain in its jar.
WALDMAN: The bodies we use in our dissecting room for lecture purposes were not perfect enough for his experiments, he said. He wished us to supply him with other bodies and we were not to be too particular as to where and how we got them. I told him that his demands were unreasonable. And so he left the University to work unhampered. He found what he needed elsewhere.
Frankenstein wanted to steal people's corpses without the consent of their families. That's a bit of an ethical no-no, and Waldman wouldn't go along with it. So Frankenstein, our "hero," got a shady assistant and went off grave-robbing. That's criminal—but nobody ever even suggests that Frankenstein should maybe be punished or go to prison or even be given a stern talking-to. If you look like a monster, people want to punish you. If you've got good looks and are related to the Baron, you never get treated like a criminal…no matter what nasty things you do. Moral: make sure your dad is a Baron, and then do whatever you want.
WALDMAN: The brain which was stolen from my laboratory...was a criminal brain.
FRANKENSTEIN: Oh, well. After all, it's only a piece of dead tissue.
Is the criminal brain really to blame for all the bad things the monster does? It's really not clear. Maybe Waldman is right and the criminal brain is dangerous. Maybe Frankenstein is right, and the brain is just a piece of tissue. The film flirts with the idea that criminals are born via bad brains. And then it flirts with the idea that criminals are made by mistreatment. But it never chooses between them; in the end, you don't know whether it's the brain that makes that monster tick.
ELIZABETH: Where is Dr. Waldman? Why is he late for the wedding?
Dr. Waldman's dead, of course, strangled by that nasty monster. So, okay—murder is criminal; no doubt about that. But the monster only strangles Waldman in self-defense; the doctor is trying to cut him open and examine his insides before killing him once and for all.
Similarly, the monster kills Fritz because Fritz's tormenting him. These are not cut and dried criminal acts; a jury might well acquit the big guy for either one. If the monster is a criminal, it's only because people attack him and…well, treat him like a criminal and a monster. He becomes what they have decided he's going to be. They made a monster, so he acts like a monster.
LITTLE MARIA: See how mine floats.
[the Monster picks her up]
LITTLE MARIA: No, you're hurting me! No!
The monster tosses Maria in the pond, drowning her. Drowning a child: that's not something you can approve of. But is it a criminal act? He seems to be trying to play with her; he thinks she'll float like the flowers they've been tossing in the water. This is both the scene which convicts him as a criminal, and the scene which suggests he's innocent, like a child. Basically, the monster is bad because Frankenstein's a bad parent. If only he'd loved his monster more, it wouldn't have had to come to this.