I think that, not even Waternoose was blatantly evil. All that he wanted was to keep his family’s company alive. They didn’t really know all that much about people there, and were ultimately scared of them. Probably, none of the monsters involved in the project, (Waternoose, Randall, etc.), thought of kidnapping a child and extracting screams as wrong. If a monster was in our world, I can tell you, that most people would not think twice about using it for experiments.
You can see Randall has reasons for doing what he was doing, since yes, we all act differently under pressure, we can freak out, do things impulsively, or be pressured into doing things that we would not normally do by our peers, or others in positions of authority. I honestly get where Randall’s coming from, but if you are applying this to him, than it must be applied to all.
Sulley and Mike, were also in a ton of stress at that point if you think about it. Mike didn’t filed his paperwork on time, (which he’s always getting in trouble with Roz for), he had a date with Celia, and she would have been mad if he went back to file it, so he asks Sulley to do it. Sulley does, finds the door, accidentally lets Boo in, and freaks out. (You have to remember at this point, that they believe that she may be deadly.) He interrupts Mike’s date, causing tension between Mike and Celia, and then the “thing” escapes, causing Celia to be in the middle of a restaurant shut down. Mike and Sulley don’t know what to do, and Mike’s crazy plans, like releasing it into the wild, clearly show that he is not thinking straight. Then, they see that Sulley’s bag was left at the restaurant scene, which means that there is something traceable, adding to the level of stress. When they try to bring her back, things do not go according to plan, and the situation escalates rapidly. Mike, Sulley, and Randall, are now all freaking out, as they all have a lot to lose.
By the point at the end, where Mike and Sulley throw Randall threw the door, they have been banished, chased, and threatened by Randall. In the adrenaline of the moment, I don’t think they were thinking so far ahead as to contemplate his chances. They all got in too deep in their own respective situations, and none of them are with clean slates and are completely blameless. Mike was wrong, Sulley was wrong, and yes, even Randall was wrong. There is a point in their chase, when they all should have called it quits. But they don’t, and at that point, like I said before, adrenaline can take over, and for the outcome to be different, that fight never should have started.
The thing with Sulley is, that he became very attached and protective of Boo. He saw Randall as a threat, and yes, he could have been partly blinded by emotion, but Randall had tried to strangle him before, so their relationship had clearly deteriorated to the point where things were becoming physical.
Mike was under stress with his job, and trying to fix things with Celia. He was often bullied by Randall, and often those who are bullied, can become bullies in certain situations. Randall clearly bullied, because he was probably pushed around by Waternoose.
Randall, had reached a point, where he was afraid that he was going to lose everything that he had worked on, so when he was strangling Sulley, he was at a point where he was desperate - and yes, he had probably been suffering from lack of sleep, but in the time with Boo, I am sure that Mike and Sulley were too.
Waternoose, like I said before, was probably coming under scrutiny from others in his family, (who were not the ones put in charge of the company), for not running it properly, so he too became desperate, so employed Randall as his engineer/fall-guy.
I suppose that you could argue any of these guy’s points, but they all did some things that are less than respectable. As for Sulley not thinking about Randall, after all that had happened, he was missing Boo, so that’s what he was preoccupied with. Also, he may not actually know what happened to Randall. The doors were not labeled, so he may of just thought that he was banishing him. Yes, it was not for him to do, but if he did it impulsively, what could he really do, except for go in after him, and say, “sorry, but I am not supposed to banish you, so please come back so that I can report you to the authorities”? (Which he wouldn’t have done, because if he threw him through a door, and then came after him, it would have resulted in more physical fighting between him and Randall, especially if Randall thought coming back meant jail time). If Randall was OK, I think he would still blame Sulley and Mike for what happened to him, and his anger would probably blind him, like it does with many, far too often.