Danzou was taught by Mito, and Tobirama, as much as he tried, was never able to quite let go of the Senju-Uchiha rivalry. Danzou doesn't hate the Uchiha clan, but Tobirama rubbed off on him, and he knows some of the things they're capable of. Kushina is amazing with seals, and he is so, so proud of her, just like Mito, he thinks, a natural, and a prodigy to boot, perhaps he's even her jonin sensei.
Kushina, as amazing as her chakra stores are, as amazing as her control is, and as amazing as she is with seals, is still young, is still growing, is still learning, every single day, and he knows it takes energy. He didn't have as much control as she did when he was her age, nor as much raw talent, he had to fight tooth and nail to be as good as he is now, and he remembers being exhausted to the point of passing out from just how hard he trained, he doesn't begrudge her for barely being able to walk, or to keep her eyes open, she's certainly fairing better than he did. So it isn't a hard decision to crouch down and let her climb onto his back, he doesn't quite care if he's seen as too soft, he knows he would have appreciated if Mito had done this for him, but she had never offered, and he had never asked.
He keeps walking, as Kushina rambles on about her theory for what she could have done better, and what she wants the seal to do, he listens attentively, she is so incredibly smart for someone with so little field experience, so it comes as a pleasant, if confusing, shock when as she's slowly trailing off into sleep, barely able to string a sentence together, that she says 'thank you Papa...' his heart skips a beat, and his eyes are so wide they're starting to water, and before he has time to think about it, he says 'you're welcome.'
When the day comes that Kushina goes into labor, he is happy for her, so, so happy, she's been looking forward to this day since the moment she found out she was pregnant, and he can't wait to meet the baby. So as the night stretches on, as the Kyuubi appears, as fires spread, and Kushina, nor Minato or baby appear, he remembers one of the things Tobirama told him, so long ago 'only a Mukuton user, or an Uchiha with a powerful Sharingan can control the Kyuubi.'
He knows, that while there is a single Mukuton user, the boy does not have enough control or knowledge to be able to do this, but there are plenty of Uchiha who have their Sharingan, and there are surely some who are powerful enough. He knows exactly which clan is behind this, and that distrust lent to him by Tobirama flourishes into a sickly hatred, it festers and twists his heart, because his daughter is now dead, and her son cursed to carry a beast that will try to control him, and in turn have others try to control him.
He will never forgive them, and he will ensure their punishment, no matter the cost.
