The deaths of the dogs toward the beginning, and later, Clark's reaction to the deaths of the remaining dogs.
Particularly the dog who made an attempt to free itself by biting through the kennel wire, and gets doused with acid for it's trouble. When The Thing starts to absorb the dogs, we see it's fur and skin is mostly burned away, but it's still alive.
When they first see the thing in the dog kennel and start shooting at it, Clark only seems concerned that they're hitting the dogs and even tries to run into the cage to save them despite the fact that doing so would obviously get him killed.
Ultimate fate of Blair. He was the first one to realise how serious the situation is and for his trouble was locked away in a shack. Not only it made him exposed (if he wasn't infected already), but the sole fact that he was there alone for few weeks, seriously contemplating suicide, is enough to feel bad for him. And then he's the ultimate incarnation of the Thing.