A cry goes up when the orc strikes - so many see it, but no one is close enough to stop it. A breath later it shot full of arrows by the vengeful Imladris guard. Glorfindel is there a moment after that, shoving the dead orc aside to check on Elrond.

Or he would, if he wasn't suddenly under attack from a pair of elves. The twins have already seen what orcs have done to one parent today, they simply cannot abide the injury of another, and Glorfindel took their weapons. Something he's deeply thankful for at the moment, they're vicious enough without them.