Revealing mistake: When Harry tells Marv to go back while they are climbing across the rope, in the shot facing them front-on, you can see Kevin in the background about to cut the rope. If you look closely this is not Kevin, it is his stand-in.
Revealing mistake: Towards the end when Harry walks up the frozen stairs, you can see it switches to a stuntman when he slips and lands on his back. (01:17:55)






Answer: Because he's understandably angry that Kevin's so unreasonably scared of him and assumes the worst in him without even getting a chance to know him. He never gets a chance to talk to him, because he always runs away too quickly. The first time he gives him that stare was when Kevin was watching him shovel from the window. Nobody would like to be stared at like that while doing a simple task.
MikeH