Why did Zeno turned his back on Yona?
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.
I'M READY FOR YI HO! BRING. THE. CUTENESS. NOW!!