This ability is bugged to where it still applies even after you kill the champion or leave it's range. I had a Dirge below 50% hp and when it moved into range of Kill Sense from a Ferren Ninja, it had the 2 ap debuff. However, when I moved out of range, Dirge still had it. I killed the ninja but he still had it.
The ability doesn't state a duration for the Slowed condition. So it might apply it permanently until cleanse, like some other things do with inhibited, defiled and so on. Slowed condition stacking is pretty weird tho.