Continuity mistake: There were only two serums left after Morbius took his. Milo takes one and kills the nurse. The next morning when Michael thinks he had killed the nurse he goes to grab a bunch of blood and make a run for it. There are still two serums, even though Milo has supposedly already taken one of the remaining 2.
Continuity mistake: At the beginning of the flashback in Greece, the pieces on and around the chessboard change. When we first see the board, most of the white pieces are on the back row, and then we see 2 pawns removed. In the next shot, when the king is knocked over, some of the white pieces on the back row have moved and there're several pieces that have been removed.
Answer: He still had a compassionate side, the donated blood was for people who had blood diseases. The reason he did all this in the first place. As for animal blood, that's only for undead vampires, he was a living vampire, who needed specific fresh blood to combat his disease. Plus, vampires drinking animal blood was made up in movies and TV shows.