Everyone wants Zed to be balanced. I have the solution!
Make the return distance on his R MUCH MUCH lower, it's fairly straight forward. He gets to all in from 1000 range away then gets to travel backwards to where he ulted from (being around 2000 units).
If a zed makes the decision to one shot champion X, he should do so within reasonable parameters! It shouldn't be as is, where you can go in do your damage and poof fly back to safety...
The reason an Assassin is overpowered isn't relative to the damage they deal (uh, k maybe just a lil bit) But how reliably they can dish the damage out in safety.
Just my 2 cents, downvote me if you please.