There was a problem running rampant among the gods. More than the petty rivalries and the jealousy and the anger over who got the bad seat for the Christmas Picture
There was the fact that certain gods couldn't keep it in their pants and then didn't want to deal with the consequences afterwards.
Gods didn't deal with their little ankle biting bastards. IT was as simple as that. So it was something practically unheard of that Hades had an interest in someone else's baby, much less his own.
Hades didn't even WANT the baby. It was just supposed to be something to keep Rumpelstiltskin reigned in. HE was the threat, not the lot of the rest. He'd have been happy to let the baby and Belle go back to the surface, Rumple just had to do what he anted for as long as he wanted. Was that so hard?
But Rumpelstiltskin HAD to make it more complicated. Because he picked up on the idea that there was a bit of tension going on between Olympus and the Underworld so he went tattling to his big brother.
And it's not like Zeus would care otherwise, but he came down to resolve this part of the problem to spite Hades. Hades didn't know why. Just because he wanted to kill him for stopping his heart and take over. It wasn't like Zeus was doing a good job, the mortals were going all sorts of insane up there.
So Rumple went to get Zeus and down Zeus came. Because while they were in HIS realm, the lot of them were under Zeus's protection since most of them weren't dead.
Zeus resolved the problem in a way no one won. Killian Jones made his peace and moved on, the charmless Charming family was sent back to their world where they belonged...but a contract was a contract.
So, Belle would stay in the Underworld to give birth and the child would live six months above ground each year and six months in the underworld with him each year.
"You want the contract so badly brother, you have it."
So now not only did he have 9 months of them being in his way, he had to deal with them pestering him for half a year for the rest of the kid's life. He knew it'd be this way, he doubted Rumple would let the tyke out of his sight.
And then Zeus cut off any possible chance of reconciliation with Zelena when eh strengthened the strongholds that kept Hades trapped here.
So just what he needed. To be the second guardian of a baby that belonged to a family he just got thrust into that he didn't want.
Hades looked at Rumpelstiltskin and Belle when Zeus left the throne room, "Now you know why I want to annihilate him so much."