Just an idea...

I really don't have a storyline for this, but it would add some good "spice" to a story. This idea would go in a sci-fi thriller, or any book/story with violance and lots of science.

Person X is a blood type O+ and person Y is a type AB,A,B (+ or -). Man X is injected secretly or covertly by person Y with blood from person Y. The type O+ in man X's blood reacts with the A/AB/B in man Y's blood, and man X dies.

This works because blood has a few types. O+ people can only be injected by O (+ or -). AB+ can be injected by anything.