I'm not actually sure that Bell's dad was so naive he couldn't recognise that Hild had some bad traits. I think Bell has difficulty understanding the dark sides of people (she even has difficulty comprehending her own jealousy) but I don't think she's incapable of recognising when people have done things she would consider wrong.

It's also interesting to note that Belldandy is very perceptive when it comes to seeing the good in people. Someone had suggested that perhaps her father had seen something good in Hild, although I wonder a bit about this. After all, other then her love for Urd, (who wouldn't have been around when her parents first met for obvious reasons) has Hild really been portrayed as having any sort of redeeming qualities? Bell's dad may be perceptive, but I wonder about him being that perceptive.

As for whether Hild loved the sisters' father... she might have, but I don't think a stable, long term sort of relationship was really possible for the two of them. I've sometimes thought that perhaps what attracted Hild to the goddesses father was that he was a frank, honest, trustworthy person who had at least some ability to understand her -qualities that she basically couldn't find in her demon subordinates, who would be either pathetic suck ups or potentially dangerous (this of course relies on the assumption that hell's politics are a cutthroat rat race). However, Hild (making some assumptions again) was also the type who's used to playing demon politics and feels uncomfortable when people can actually read her (she likes to keep the people around her off balance). So the fact that the goddesses father could read her and be honest with her may have have been important for their relationship, it may also have kind of doomed it as well.