This is total supposition, but I think it'll come out that the woman involved may have been in a relationship with someone else when she got pregnant with the now-deceased baby. Mr. X then found out the real father of the kid he thought was his was due to her cheating on him with her bigshot NBA ex, lost his mind, and killed them both. This would explain why the two of them were murdered but the older child (who predated the relationship) was left unharmed. The whole situation sounds like a sheer revenge killing against mom and baby, and the most likely reason is someone felt betrayed.
Either way, it's a tragedy when something horrible like this happens. No matter what anyone involved did or didn't do there is no reason to execute a helpless baby like that, not to mention the surviving child might well have witnessed some of it. My heart goes out to Curry, the surviving child, and those who were close to the victims.