Now, let's take a look at his probably self-defense technique.
Based on the x-ray, it appears as if the blade was embedded in his bicep or tricep area of his left arm, entering above the elbow, with the tip facing his rotator cuff.
Given that most people are right-handed, and the victim said the man tried to stab him in the chest, it would seem as if the attack was a thrust, blade held in the classical style with thumb on top and cutting edge down, in a rising uppercut-like movement towards the chest. The victim, then, would seem to have tried to sweep (I believe he said 'deflect') the knife with his right arm, away from his body. Unfortunately, it did not clear his arm and he took the blade thrust there.
To me, this seems to be what happened, and if it is (sorry for the Monday morning quarterbacking) then he did everything right. You always move the blade away from your body, never towards it. The blade extends the attacker's reach, which makes it harder to sweep if you're used to dealing with an incoming fist only.
Just my 2 cents on a Monday morning.