I don't think there can be any meaningful dispute that looking for a "black b******" in the hopes of killing him, ie, basing his decision and motivation on skin colour, is anything other than massively racist. I *still* agree that it shouldn't be held against him, given the shame he expressed, but to claim that it's not racist is somewhat of a stretch, to say the least.
Also, I should just add that there is a difference between an incident being racist and the person being racist. Neeson's actions and thoughts at the time were highly irrational, and motivated by a sense of revenge and totally misplaced justice. That doesn't make him a racist in day-to-day life, but what he did -- or what he thought about doing, what he *hoped* to do -- there can be no more racist thought than wishing to kill a black man, any black man, because someone you know was raped.