
I think it's because as a reader of this genre we "know" that the FL will be safe and the beast can't harm her.
BUT as a character living in that world, there is no way the 2nd ML knows his treasured friend is safe - forced marriage with a beast and all. It is very understandable that he will try to safe her. It would be more strange if he let go without even trying to get to her.
If your friend is in possible danger, will you try to help the best you can or will you abandon her?
Imo, his heart is in the right place.

The author just named the so-called secondary gender "dom & sub" as if it would make it ok to force the "dom"s will on the "sub" without his consent. Especially so when the other party is helpless to his "orders". Consent is big in proper BDSM relationships. Safe words, agreement on the limits between the parties involved is required.
At least omegaverse has an excuse of pheromones which makes alphas lost their minds, but this asshole incel has all of his faculties and acted as if he was doing he world favor to rape and manipulate the MC. Which is creepy, disgusting, not sexy at all.
I'm dropping this.
I hope she can find it in herself to ask for help to the right person before it's too late :'(