What happens if you're injected with blood of an incompatible type?

The blood would clot and organs would start to fail, you would not be able to urinate and you would eventually die, unless you got the right blood in time

i think your immune system would just ill all of the bad blood and nothing would really happen.

It starts with chills, aches all over, shortness of breath, red cells breaking up and free hemoglobin from these red cells coming in your plasma, filtering down to the kidneys and damaging them, which can eventually lead to kidney failure.  Since red cells will first clump together it will form clumps in the smaller vessels, occluding them, if enough of the lung vessels are occluded not only you are very short of breath, but blood pressure goes down, you loose consciousness and die.

If the amount is significant, your antibodies would attach to the antigens of the injected blood and cause a hemolytic transfusion reaction.  This lyses your red blood cells.. thus you become anemic and your body cannot recieve oxygen (your RBCs carry O2 to your body).  This causes you to die if intervention did not come in time.

A Haemolytic Transfusion Reaction may result from exposure to foreign red blood cells that are A/B/O incompatible. The victim' must be group O, A or B. Group AB cannot experience this type of reaction.

The victims antibodies attach to the injected red cells and the complement system is activated, culminating in the destruction of the injected red cells within the blood vessel and the release of haemoglobin into the plasma.

The victim may experience systemic hypotension, Disseminated Intravascular Coagulation, renal vasoconstriction/intravascular thrombi (clots), leading to shock, renal failure and 10 % of patients in hospital with this reaction die.

Victim will be in pain, probably back pain due to kidney effects, they will get the shakes, go very pale and weak, and sweat a lot

About 50 ml of red cells would be needed to achieve a reaction but:

1. The victim must be Group O,A or B
2. You must inject >50ml directly into a vein, which could only be achieved if your victim was incapacitated
3. Chances are the victim won't die, or will die very slowly.

All in all a very ineffective way of killing someone

Just overdose them or bludgeon them! Simple.