Lethal Duelist, Variant (5e Feat)

Prerequisites: 13 Strength or 13 Dexterity
You've learned to aim in vital points with your weapon. You gain the following benefits:

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee attack as a reaction.
Before making a melee attack with a weapon where you are proficient that doesn't have the heavy or range properties, you can choose to take a -3 penalty on the attack roll. If the attack hits, add +6 to the attack's damage.