The last drop and “You`re fired” When?

> what is that immediate red flaf that forces you to act and fire the person?

Immediate firing should be reserved for severe situations like breaking a law while at work, fraud, sexual harassment, physical violence etc.

For anything else like performance and personality issues, you should have a systematic way of giving feedback and working through these problems so that if you ever fire anyone, it doesn't come as a surprise to them. And if you ever fire anyone for something like low performance and this is surprising to them, you as a manager haven't done your job well.

I used to say follow your instinct, but there are a lot of situations where this has been wrong. Some people are brought up to be abrasive or hardheaded. One person we interviewed insulted the interviewer (which was grounds for an instant rejection). For some reason we hired him anyway, and he was the best performer on the team. He held some rather racist thoughts and distrust about us but after we talked it through, he was cooperative.

Another near instant firing moment was a teammate who just didn't get it. He'd put bugs in and insist they were part of the design, or that they were someone else's responsibility. He'd be completely stubborn and it would take two people explaining the situation to get him to admit fault. But this was early in the job, where he was insecure, facing a lot of trouble at home. After we made it clear that bugs are the whole team's responsibility and not one person's, he was a good core member of the team.

So those are false red flags from my end.

But you can still spot real red flags from a distance. I think there's rarely ever a "immediate" red flag. Even things like harassment and racism might not be on purpose. Firing someone for racism might actually be counterproductive. There's a touch of racism in many companies - certain races/religions get paid less and "talked down to". And some of the more privileged races (not always white) might be distrusted. Usually this can lead to a subtle culture where someone acts like an asshole because they don't want to be pushed around. But if you show them a little love and care, they can often turn out friendly and they'll usually become far less racist.

"as soon as possible."

You dont sound like a top manager to me