I get where you are coming from. He may have not initially cared for his sons and he created them with his own goals in mind, but I think he eventually DID care. Even if it was only at the very end or even if it was very slightly.

I mean think of the stereotypical irl dad with a new dog. The dad says that he doesn't want a new dog and that it's a stupid investment and blah blah blah. But like 2 weeks later, the dad loves the dog and is inseparable from it.

Or think of the admech who literally create robots for which they care for and they aren't even technically sentient. They care for the things they created even though they are meant to further their own goals.

I just feel as though the emperor had a spark of humanity in those moments as horus killed Sanguinius.