Why did Zeno turned his back on Yona?

Because this whole time his plan was to end his immortality and finally die.
It's quite understandable too, immortality sucks.

Further, I think the only way he can die is if Yona kills him.

So to force her hand, he will kill Hak.

I hope he won't kill Hak :(

Just ask Yona to kill him to end his suffering

Fuccc I'm scared to continue reading this :(:(

... im gna kms aft reading ur comment :<

That's the problem. She won't do it without provocation, or so he believes.

If taking away the other three dragons and forcing her to eventually take the throne against her will doesn't cause her to act, he probably thinks killing her lover will.

But this is all speculation. For all we know, the visions we have of Hak dying/being killed/being wounded are unrelated.