I know, and that is why I have not accused her more directly. The alibi is the problem. But there are many things about this that fit Undyne's particular skills quite well and not anyone else's. It's the lack of a murder weapon but the fact that it must be a larger blade that catches my attention the most, but her strength makes it more feasible that she could knock Gilbert out as well as... Obscuring her blade's mark, shall we say.