I thought the same! I don’t think Zeno is the bad guy here. I was thinking he is sealing the 3 dragons before they go out of control and lose their conscience; and release them once they have cooled down.
Yeah but why being so sneaky about the fact that he has the chalice?
Hoping and praying that this is the case
Hoping and praying that this is the case and that all my tears right now will be for nothing and that we can all laugh our silly little laugh when we realise we were all bamboozled
Lawd this didn't age well, I still hope that by them being in the chalice (and Yona sensing that they're still alive they still could be saved)
Zeno sealed Jae-ha, Kija, and Shin-ah not because it's the end of their lifespan. That chalice is really important to Zeno not just because it's what gave them the powers, but it can actually extend their lifespan. There are a few theory I consider about this,
1. Zeno sealed the dragons so they can recover, both their souls and their bodies.
2. Three of the four dragons aren't going to die young this time around because the reincarnation of King Hiryuu has descended. The first generation of the dragons all lived until old age, the descendants died because King Hiryuu wasn't in their generation(s).
3. Zeno would release them back when they're ready or when the time has come for their release.
4. As Kija felt, Jae-ha and Shin-ah are still alive, it made no sense that the three dragons are dead or waiting to be reincarnated again because Yona would sense it. Zeno looked relieved because he sealed them before they died or lost themselves.
Again, this is just my theory. I hope it's not too far off.