Ichigo learned, after his son's (fuck, his son- he's still having a hard time wrapping his head around that one... despite how much the little kiddo looks like him) rather distressing birth, about not only Nnoitra's disturbance on his way to the castle, but also Ulquiorra's bat nose sniffing about what would later be Grimmjow and his kid's temporary bedroom.
He didn't have much time to process any of it, not the birth, or the fact that he was now a father, or Grimmjow's weakend state (given that his body was definitely not made to birth an actual child, magick uterus or not), or even how Grimm had called the little bundle 'our baby' before he fell into well deserved slumber, or all the thoughts that entailed; but one thing that was sure to him was how enraged Aizen would be.
He's dealt with him before, the man has a grudge, an obsession with his family. For this to happen would only set on fire the begrudging peace both sides had quitely held onto this past years; and he's realized recently that's something he's just not willing to lose.
Aizen tried, quite directly, aggressively, to enter the castle. He wouldn't believe it true if only he hadn't realized how much this meant to the moth those first few days after the birth. The rage, the violent outbursts; the way Aizen called out Ichigo in front of the town for 'taking away what was rightfully his', putting the end of his species on his shoulders.
Ichigo can't say the look in his dark eyes didn't shake him to his core. Family has always been a sore subject for him, but for that very reason, Ichigo simply couldn't overlook the way Aizen had used the witch. Much less could he forgive the ease with which the man would've slain his child hadn't he been stopped; despite how the child's very existence had only been possible due to him.
Ichigo could be merciful, but if losing an entire bloodline of ancient creatures meant his newfound family would be safe... he'd end it himself.