Ygor promised Bohmer that he would give him anything he wants, make him the regent at the university, the head of the medical profession in the state. "We could rule the state, and maybe even the whole country," Ygor said. Bohmer obviously fell for Ygor's promise because he knew the monster with Ygor's brain would be a force to be reckoned with. Ygor would become "a leader among men," and possess the strength of "a hundred men." Bohmer, in his mad dream for power, did not consider the blood issue. After all, you have to remember he made "a slight miscalculation" before during an earlier brain transplantation experiment.
"I cannot die! I, Ygor, will rule the world! ... I should kill you, Frankenstein, but after all, your father gave me life and you gave me a brain!"
Good point. In the 2nd movie a group of people were able to capture the monster and although he escapes he probably would have been captured again if not for Dr. Pretorius harboring him.