if you want to believe that iain being a dickhead towards ruby was because of his mental illness then that's fine so long as you acknowledge it isn't an excuse but the writing itself pushes the idea that he was consciously holding an unwarranted grudge against her so, like, it's perfectly fair to say that he didn't have a reason, that his behavior is unjustifiable, and/or that specifically targeting ruby did all but come out nowhere, particularly from a writing standpoint.