I agree that manhunt is not a game that deserves defending. At least in GTA you didn't have to be violent against civilians if you didn't want to. Gang war is one thing, hunting pedestrians is another. Manhunt however has no other option.
However i am concerned with the generalisation of "banning violent games". I'd say 99% of games are either violent or have an option to be violent or can be reinterpreted in a violent way (supermario). So banning violent games would be banning all of them. I think it would be alright if shops stopped selling mankind putting pressure on completely obscene games, where there is no moral behind the actions or the story and no option to be at least partly decent.