I'm not sure as I haven't read the 'killdozer short story', but it could be the bulldozer operator as a main character. There might also be other people on the site like supervisors or workers who are affected by the events involving the bulldozer.
The main characters in the 'killdozer short story' might include the operator of the bulldozer, who is in control (or perhaps loses control) of this large and powerful machine. There could also be a project manager or a safety inspector who becomes involved when things start to go wrong with the bulldozer. Additionally, if the story has a more human - interest angle, there could be a family member of the operator, like his wife or father, who is worried about him and the situation with the bulldozer.
Since we don't know the exact plot of the 'killdozer short story', it's hard to say for sure. However, the main character could be the owner of the bulldozer who is facing some sort of crisis related to it. Maybe there's a mechanic who tries to fix the bulldozer when it starts acting up. Another possible main character could be a bystander who witnesses the strange events involving the bulldozer and has to take action.