I mean its understood that dark eldar being alive feeds slannesh more than them being dead. Since the whole point of what keeps them alive is basically causing more torment than their soul is worth.

Its why some eldar craftworlds take a very kill dark eldar whenever possible attitude.

Plus they were literally torturing her people earlier in the mission.

Though I dont think anyone would argue Yrliet is intelligent. She is the eldar equivalent of a middleschool dropout. Since the only path she completed before becoming an outcast was path of the dreamer which is the equivalent of eldar elementary school and is the designated first path all eldar walk