He's a decent shot, but they are not hard shots to make. Where he is clever is making sure that he's not noticeable before the shot is fired.
If a terrorist was cut off from his support group,these shootings would be cheap to carry out and could be easily planned and carried out by one or two.
I've been expecting sniper shootings for some time by the terrorists instead of the constant suicide acts.They must be able to count and see that they need everyone of them to carry out more than one attack,or they'll shortly run out of pawns.
That said,yes,we have plenty of just plain crazies that could be doing this.