When Alice heard Alister's words, she realized her mistake of being too desperate to live before that she had offered to be Alister's servant. She had even seduced Alister before and when she realized that, it brought her an even greater shame!
Noticing Alice's embarrassment, Alister didn't tease her any further.
It was only when Alice finally recovered from her embarrassment that she asked Alister, "You should have known that I wasn't lying when I told you that you are my brother. But why did you kill me?"
Alister already expected this question so he answered fluently, "I don't want any unstable variables in my plan. My survival plan was only with Lynn and you weren't included in it. I couldn't afford any changes to my plan since that timeline was very crucial to me as it determined my survival in the next timeline."