We didn't even see the real Grayson sadly cause of such a bad Adaptation, the real reason he left was that the situation and fire was very serious and Grayson was telling Dane to turn back but Dane didn't turn back, Grayson was planning to drag Dane back by force but we all know Dane he ran away leaving Grayson alone, also Grayson had an Order from willkins that he must obey all commands of dane and when Dane told Grayson that he is a liability and told Grayson to go out he obeyed Dane and house started to collapse so Grayson did what any rational person will do.
It's not that bad honestly as people are exaggerating, I feel Dane suffered the same amount when Grayson leaves Dane after Grayson breaks up with Dane which leaves Dane completely heart broken and full on depression. As for physically I think Dane suffered the most with a lot of emotional problems and depression by the end, as for Grayson it's more on the point of just longing for Dane's love so it's not much suffering if we scale both suffered equally
I think we can call both had chasing arcs like half of the novel Grayson was longing and chasing for Grayson's love and by the end it was Dane chasing for Graysons love (but Dane never acted on it but his feelings did say how much he longed for Grayson and loved him by the end) so u could say the whole novel itself is a chasing arc.
As for Dane I think he deserved whatever came to him lol I was actually enjoying Dane depression arc

I need Greyson to suffer on levels that no top has suffered before. That man does NOT deserve Dane