Can someone explain monomachy to me?

So the description reads:
Monomachy
Increases damage dealt to target by 10% while lowering damage taken by target by 10%. Effect ends upon executing action on a different target.

So basically... it should be renamed kill steal? All it does is lower the damage your team mates do while having no positive effect on you? (10% damage increase and 10% damage reduction cancel out).

Why would SE give them this skill? At best it's a dick move at worst you could troll with it. Is the tooltip an error? Am I missing something?

It's a bad description I think. You can read the debuff it puts on the enemy to get a more clear one, but here it is in a nutshell. As far as I can tell, at least.

It increases the dmg you deal yourself to the target by 10%, and decreases the damage done by them to you by 10%.