As distastful as this line of thought is, he'd also probably be intelligent enough to know that someone staking out Woolwich barracks probably weren't waiting for him specifically to emerge. Your last line is irrelevant anyway, as clearly the context here is meant to be an attack in Woolwich, not on Woolwich. Although arguably waiting outside Woolwich barracks to kill a random soldier is indeed an attack on the district of Woolwich (with the choice of barracks being as random as the choice of individual).