Its not damage and shit he needs his W is fine, he needs his E to be like a dash with something added on top.
He needs a lower CD on his Q just a base at all levels.
Lastly he needs 1 sec of invincibiltiy and immunity to cc AFTER his passive so he just doesnt insta die after its done.
Also maybe something on his ult to make him a bit more durable in a unique way, maybe not so straight forward like tankiness but something interesting and out of the box.
Like he cant take more than 10% of his max health in damage in one single attack, meaning if he gets into a duel with rengar he will actually be able to sustain instead of getting taken to 30% hp instantly then getting one heal off and insta dying again.
Or maybe even make it so enemies he actually HITS with massacre do 50% less damage to him for the duration, meaning hes gotta jump in the middle and use it, making his style of all in more appealing.