There are innumerable possibilities. She is old, so the girl could simply be mad.

[ but that's a non-answer, so: ]

Many of us have matters that need attending to. She may have killed to return — a plight I can sympathize with. It may also have been purely incidental. Humans are fragile, and she may have found herself in a mess that she could not untangle — a smoke and mirrors murder may have been the simplest way to solve this. In the end, does her reason convince you more or less, Buddy?

[ genuinely curious. not being condescending, for once. ]