
I have at least three hypotheses:
1 - Brett is being violent and stubborn on purpose, so that Hayden takes out all her anger on him during heat (we have to remember that Brett is addicted to strong emotions). Maybe he's too full of himself that it will be so good for both of them that he thinks that after sex Hayden will forget the demand he made before.
2 - Brett is going into heat too and that's why he's acting that way. It's like a battle between alphas, who want to violently possess the other in heat.
3 - Brett is a worse person than we thought and when contradicted, for the first time in his life, he couldn't bear to be discarded. as he realized that he cannot manipulate Hayden like the others, he turned to violence.

The funny thing about Brett is that I think all three of your hypotheses seem correct. But thinking again, didn’t we go through four of Hayden’s ruts while Brett didn’t have any? Could it be that Brett’s and Hayden’s ruts were assimilated, leading to Hayden’s rut also being Brett’s? Quite interesting.
As for the last point, I think the beginning of the season2 has already proven this, Brett is cruel than we thought.
What I’m afraid of is that season 2 will turn Hayden into a helpless victim of Brett. If that’s the case, it will be hard for me to support this OTP.
Interesting parallel I saw was Brett tackling Hayden in the same manor Hayden did in the very beginning of the manhwa. It’s making me think that this rut is going to be a big turning point of their relationship. Though im unsure what they’re saying, I hope Brett gets his shit straight though. I have the impression that he still is holding onto that previous agreement, but if that’s the case I hope Hayden draws a stronger line between them and fully shows that he means what he says. Brett is kinda hard headed