Just to clarify, you do understand it was not the "cornered/threatened" LEO that shot him (in the head) - don't you?
Where the shot landed is not important. In a court of law a shot fired is a shot fired, there is no warning shot, no wounding shot, or otherwise any shot fired that will not be construed as being than that of one that was fired "to end the situation". The issues that come in to play are "how many shots were fired", and "when where they fired". If the perpetrator had been hit in X body part and been thrown from the chair on to the floor and the LEO kept firing then sure there are grounds to call for "use of excessive force". In this case, if there was one shot fired and that once shot hit the perp in the head it is highly unfortunate for both parties involved. We have 1 person dead and another scarred for life.
Also, if the LEO was cornered and was unable to use proper sighting techniques for fear of exposing his gun to the perp, it is quite possible that a shot was fired using a non-traditional stance and/or grip. For instance, we were taught then when in "tight quarters" (within arm's reach or a few steps away), to not fully extend the pistol out for obvious reasons. We practiced firing with the gun close in and tight (under the arm pit, from the hip, across the chest), and from many positions (standing, kneeling, sitting, prone on side/back/stomach, etc). Oh, and we practiced using either hand, too (using the "off" hand is difficult enough while using a supported stance, doing it under stress while getting beat on from a odd position is another).
My point is, none of were there and none of us saw it. I caution everyone to be very careful rushing to judgement of either the [evil power abusing] LEO, or the [poor defenseless wheelchair bound amputee] of the perpetrator. As usual there are far more things we do not know about the situation than that of the "facts" we know. Remember that, it holds true is a vast majority of things in life.