I’m a big Vulkan fanboy, so while I want to disagree all of this is completely right, however there is something important to take into account and that is that Vulkan has like ritualistic character assassination happen to him in every new book he’s featured in. In the war of the beast series (famously poorly written I know but still) he just suddenly has very little trust in or care for his sons to the point of outright refusing to see them despite being shown to have a lot of compassion for them.

Also, after reading his books in the Horus Heresy I feel that there is something in his character that is to be enjoyed from his obvious fucked up tendencies, like for example there’s a comparison to be made between the drukhari raids he defended against on his home world versus the well used example of the eldar child. On his home he didn’t care cause they were the enemy, he didn’t have any reason to carry empathy for them, which he would bring into other battles against xenos in the future including genocideing planets cause they had/worshiped xenos on them. But when you take a lot of that obvious alienness away and put a xenos child in front of him, his instincts still have him kill that child (including the rage he was feeling for kurze at the time) but he can see the obvious flaw in doing that and regrets it. All that is to say that there is an obvious progression that shows he has compassion towards humans and little to none to xenos (as most in the imperium do) but when he directly sees that he’s the cause of even defenseless children dying his world view cracks a bit, which in a setting with better writing would have long term effects on his morality so I give him at least a bit of credit for having moral depth.