The reasons could be complex. It could be related to her psychological state, financial problems, or even a tragic misunderstanding. Without more details, it's hard to pinpoint exactly why she did such a horrifying act.
It's likely that Merricat's actions were driven by a combination of factors such as her own mental instability, past traumas, and a desire for independence. Her family dynamics might have pushed her to this extreme act.
It could be that she was under extreme stress or had suffered abuse from her mother, leading to such a tragic act. Or perhaps there was some external factor that influenced her actions.