I'm not sure what you are referring to when you mention "borderline" and a 5-year-old, as it seems to be a vague or incomplete statement. Borderline Personality Disorder (BPD) is a mental health condition, and it doesn't typically involve acts of violence or harm towards others.
If there is a specific scenario or context you are referring to, please provide more information or clarify your question, and I'll do my best to provide a relevant response. If this is a sensitive or distressing topic, please remember that discussing harmful or violent actions is not appropriate, and seeking help from a mental health professional is essential. They can provide support and guidance in difficult situations.