Question

I think because of his brother? just gut

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When Chase was in crazed state rut due to excessive pheromone built up he slept with Josh, he doesn't remember it and so Grayson took advantage of that and manipulated Chase into thinking that he slept with a dog. Grayson did this because Chase have been very adamant into not attending Grayson's orgy parties or even having sex with an omega which caused his pheromones to build up dangerously. Grayson seriously thought that by traumatising Chase into thinking he slept with a dog during his crazed state rut, Chase would be more keen on having regular sex with omegas to release pheromones and avoiding the same incident from happening again.
Grayson do care about chase but he show it in a very f*cked up way. From my understanding of the novel, it seems that all of Chase's siblings are pretty much psychopaths.

oh damn, because of his fcking brother ಠ︵ಠ

It was his first rut, not the one with Josh.
And Grayson actually brought his dog Alex, because he paniced, but nothing happened. Chase passed out and he used an injection. The problem is that Chase does remember next to nothing, but that there was a dog present, and no one told him the truth (at least their alpha father is also aware of what happened, and was mad at Grayson, but that's not in KMIYC).
And Grayson, who probably thinks that a dog as a last option is a possibility, uses that to force Chase to attend those pheromone parties, since Chase is not willig to get rid of his pheromones in any other way. Those pheromone parties also have medical staff present, so they are also the best option to get rid of pheromones by injection if they don't want to have sex and don't want to go to the hospital.
But I don't think that Chase is even aware of that, since he always passes out.

Would Chase have slept with a random omega without that trauma? I'm not certain about that, becasue I don't know how much his aversion for sex without feelings would habe been an issue then.
At least when a Miller really falls for one, they tend to go for injections, if they can't release pheromones with the one. At least 2 of them went that route, one (the father) for a decade.
All dominant alphas are psychopath, it's just the ones who manifest later, after puberty, are much less affected than the ones who manifest early. 4 of Chase siblings ( dominant alpha ones) manifested before they were 5 year old. And Grayson ( and his fraternal twinsister, still not certain since they are treated differently) was born manifested.
Chase should actually still have blue eyes during that chocolate commercial as a child, don't know why the artist colered them violet.

can I ask, if grayson really hate chase? if yes, why?

When Grayson was asked by Josh why he ventures into Chase’s life so much when this is the result and he said “it’s because he’s my brother” I fucking bawled


Also I don’t think chase would’ve slept with an omega he mentioned once that pheromones from omegas are sought out and seen as influencing feelings/affection. So I think he would’ve comprised himself at that time if he did that. Later on when Josh helped him understand that pheromones aren’t the end all be all he was led to believe.

He doesn't hate Chase. Chase is his little brother who needs to be taken care of. It sound strange when you first read this, but it actually is the case.

No matter how messed up it is, Grayson wants to make sure that Chase releases his pheromones, and that is really important, because pheromone accumulation is really dangerous for dominant alphas.

The problem here is that dominant alphas are all psychopaths. And Grayson was born manifested, so the extremest form.
As children, the 4 who manifested early, you get the impression that they have problems with understanding right and wrong, somehow the twins got the idea that throwing Chase down the stairs would help Chase. And they where sincere about that, Koi their omega father thought that, but also tried to make them stop, since they hurt Chase.
It wasn't just Grayson, his twinsister Staseems to have big part in it.
Grayson almost killed his omega father when he was under 10 years old. They, I think because his twinsister was always involved, wanted to test if their father really couldn't smell. Grayson baked a cake with detergent, Koi ate it and got sick. The children didn't help, and the twins asked questions as it was an experiment. Ashley, the alpha father, almost killed Grayson. The oldest brother was also present. It looked like they didn't understand at all.

The alpha father after that used drastic measures raising the children.

With Grayson you also have the problem that he can't understand emotions, is unable to recognize them. He was trained by teachers, so he can read people, but still not perfect. That's why he often comes of weird, his reactions unnatural and sometimes completly off.

So he doesn't hate Chase, and in his own way cares for Chase, but his way of helping Chase it problematic. His helping in general is.
He often is convinced that the way he thinks that helps really does help, but it can be the complete opposite, but Grayson doesn't recognize it.
And you see that Chase got trauma and hates his brother, because of that.