Kill Stealer is an Offensive Upgrade.
It is the upgrade version of Targeting CPU offensive Equipment item. You do +10% damage to enemies not targeting you. It is one of two earliest upgrades used by Evelynn, which goes well with her other skills (she is a Ninja and has Chameleon trait, which gives her ability to come close without being spotted by enemies). It can be also used by Jyn, the sniper.