Today out of curiousity I ask my dad, what will happen if we accidentally give B+ blood group type person, blood of AB+ blood group type?! My dad said person will die in half an hour or 15 minutes because of blood incompatibly. After listening this, Guess what came in my mind!!
What if we want to kill our enemies or someone and we know what's their blood group type, we can just inject them with another blood group type that their blood is not compatible with without their knowing.
And I think no one will know you are the one who killed him.