I've noticed there are a couple of posters/viewers who believe that Pierce was the one who strangled Clean Willy, although we do see Sean Connery's character was seen by Willy and that's why he runs, we don't ever see Pierce actually chasing Willy as he runs through the city streets. I believe it is one of the older henchmen in Pierce's gang who kills Willy. If you stop the frame where the killer is standing outside the building after doing the deed you can clearly see the man is definitely older and shorter than Sean Connery is and he is not dressed as well, he also is not carrying the cane that Connery was holding earlier in the tavern scene. It is more likely that Pierce was the unseen person distracting Willy with the object falling down the stairs. Check it out! Great book! Good movie!