Not honestly that controversial, since it's not an "instant death" attack.
Instant death implies from full life, all yeu have is essentially cho'gath's ultimate as a passive, except without a cooldown and no bonus health or stacks and such.
If a champion has 3000 health, which's a good chunk, this essentially means he gets one single 300 damage nuke that does true damage, if he's in melee range. Not that bad, but 300 true damage for free is a lil bit high still. There's also the matter that it's kinda bland and boring. The player doesn't really choose when to use it, and has no real control over it. However, both these issues can be fixed easily.
Quick and easy fix? Make his Q, *REAPING*, be required to have the bleed on the target for the instant kill to take effect. Even build it directly into the Q and give him a new passive.
Q: Reaping: Qyalis swipes at target enemy, dealing physical damage and causes them to bleed for 5 seconds. If the target is attacked by Qyalis while bleeding, and is below 8/9/10/11/12% of their maximum health, they automatically die.
Tada, easy fix, and balances it up nicely. Makes it a nice reason to put more points into the Q early, without being overly strong, since yeu both have to use Q *AND* a melee hit after, and prevents yeu from killing 5 people at low life, instead only being able to use it based on the cooldown for Q.
Nothing controversial about that at all, really.