The chief attendant tried to get rid of Lin Fan first. She did it very discreetly, leaving no concrete evidence. Even if she failed, Lin Fan couldn't say it was her doing.
However, she miscalculated. Lin Fan had no need for reasoning because Lin Fan had already set a trap for her. When she poisoned three Death Warriors, Lin Fan had arranged for the incident to be recorded.
With this irrefutable evidence, what need was there for reasoning? From start to finish, Lin Fan was just toying with her. After getting the recording, Lin Fan did not immediately expose her true identity, simply because he wanted to see what she would do next.
Now, Lin Fan had figured it out: this chief attendant is a master of poison. She's very likely to poison everyone on the plane, which would still achieve the aims of that group — to eliminate the target.
Now that the evidence is concrete, there's nothing more to say. In the face of evidence, no justification holds any weight.
