Okay, to explain your situation...

Player X was stood next to Player Y.
Player Z (You) killed Player Y because he commited a T Act.
Because Player X didn't help you kill Player Y / didn't act on the KOS, you killed Player X.

You cannot kill players because they didn't act out on KOSes, if he didn't ID the body for a solid 3 seconds, that's a T Act, however in this situation you killed him instantly. The reason this exists is that is someone is AFK and a KOSed person walks past him, you can't then kill the AFK. The slay was valid as you killed him on Suspicion.

Okay, to explain your situation... Player X was stood next to Player Y. Player Z (You) killed Player Y because he commited a T Act. Because Player X didn't help you kill Player Y / didn't act on the KOS, you killed Player X. You cannot kill players because they didn't act out on KOSes, if he didn't ID the body for a solid 3 seconds, that's a T Act, however in this situation you killed him instantly. The reason this exists is that is someone is AFK and a KOSed person walks past him, you can't then kill the AFK. The slay was valid as you killed him on Suspicion.