What did the screenwriter want to achieve by that?

Slavi is using techniques to intimidate et take power over the other person. He's the Man, the Power-that-be, what he says goes. And you'd better understand that quickly, of fear the outcome.

Interrogation techniques use different aspects of mind-manipulation (hot-and-cold also known as good cop/bad cop, Reid technique/FBI, spies interrogation...).

You can break some men by bullying, on others this has the reverse effect. Decide beforehand whether you are likely to obtain results with the subject you are dealing with, either by bullying, by sarcasm, by cold and impassive treatment, or by sympathy and by working on his emotions (Lieutenant Colonel Oreste Pinto, MI5 interrogator)

In this scene, Slavi is doing just that: he shows interest, empathy, sarcasm, then he tries to instil fear by quickly switching to intimidation. He's blowing hot and cold. The big gap between listening and showing empathy then roaring at Mc Call has a goal: take power by intimidating. I can be as bad an ass as I can seem nice. He rejects all points, and gives his. I'm the Boss, get out now!

That's when Mc Call decides that Slavi has made his point and there's no more discussion. Action!

What did the screenwriter want to achieve by that?

Slavi is using techniques to intimidate and take power over the other person. He's the Man, the Power-that-be, what he says goes. And you'd better understand that quickly, of fear the outcome.

Interrogation techniques use different aspects of mind-manipulation (hot-and-cold also known as good cop/bad cop, Reid technique/FBI, spies interrogation...).

You can break some men by bullying, on others this has the reverse effect. Decide beforehand whether you are likely to obtain results with the subject you are dealing with, either by bullying, by sarcasm, by cold and impassive treatment, or by sympathy and by working on his emotions (Lieutenant Colonel Oreste Pinto, MI5 interrogator)

In this scene, Slavi is doing just that: he shows interest, empathy, sarcasm, then he tries to instil fear by quickly switching to intimidation. He's blowing hot and cold. The big gap between listening and showing empathy then roaring at McCall has a goal: take power by intimidating. I can be as bad an ass as I can seem nice. He rejects all points, and gives his. I'm the Boss, get out now!

That's when McCall decides that Slavi has made his point and there's no more discussion. Action!