Ekko's Ult

Just before I start typing anything else, I'm going to say that I don't mind Ekko's kit at all. It maaaay be a little bit crowded but otherwise I don't care for it.

In a game as Kha'Zix jungle I noticed in mid lane that the enemy Ekko was sitting there farming with no HP or mana due to (unfortunately) killing my mid laner. So I come along to easily kill him or make him burn his flash so I can come back later to gank him with my mid laner. As soon as I jump in, these things happen:

1. Ekko starts attacking me and trying to run away
2. I slow him and burst him a bunch so he's almost dead and I can AA him to end him
3. He ults and gets his HP back and damages me
4. Since I used my abilities I can't do as much damage and I have almost no HP so I'm forced to back off
5. Ekko walks off with my flash burnt and his flash still up

After this trade I was annoyed at my stupidity of forgetting that his ult cost no mana. But really, should one ult be able to heal, burst and teleport for 0 mana? This has happened twice now as being Kha'Zix, he's almost dead, I jump in and burst, he ults and I'm forced to back off since I used my abilities and can't kill him without him killing me in the process.

This is sort of a rant at how Ekko's ult costs 0 mana and burst champions deal all of their damage but have to wait after he ults before they can burst him again. And in that time Ekko can probably kill them. I'm fine with his kit, I feel outplayed when Ekko Qs me, jumps to me and then kill me. But when come and try to kill him, he just ults? For free? He gets HP and damages me for 0 mana? I feel cheated when that happens. I should get the kill if he has no mana and no HP to fight me with. Not press the R button and start AAing me to death.

EDIT: Let me just say that I saw Ekko's ult far away from him when I jumped in, and I know that I need to stay away from it or I'll get bursted. I thought I could kill him in 3-4 seconds, but it came in 2 seconds and he used it so I had to go away.