When killing an NPC, it is possible for the Dragonborn to receive a letter from themselves for doing so, rendering WiKill04's quest unable to complete as you cannot claim a reward for yourself. This is a known bug from
https://en.uesp.net/wiki/Skyrim:An_enemy%27s_gratitude
and was recently reported on the USSEP comments page, so it still exists. This can be rectified by adding a new condition to the Enemy Alias that the Subject of that alias is not the player.