Killer, Insane Murderer

Whenever a creature is dealt damage by Killer
and dies, put a level counter on him.
( 1 - 4 Levels ) When Killer becomes tapped, he
deals 1 damage to target creature.
( 5 - 10 Levels ) When Killer becomes tapped, he
deals 3 damage to target creature.
( 11+ Levels ) When Killer becomes tapped, he
deals 6 damage to any target.