Karloff as a madman is always entertaining to watch. This time around, rather than seeking revenge on those who've wronged him, his character is unable to control the homicidal urges because of the serum injected to his own heart. Therefore, he lives with the 'poison' of a killer’s blood in his body but given that his homicidal urge is accidental, he remains sympathetic throughout. For such a simple plot, this is a pretty effective film. It looks stunning and there's a lot of emotion between the lines. It gets very dark in some parts and the quick pacing makes this overall an enjoyable effort. 4th film from Eureka 'Boris Karloff at Columbia' set.