If you use the knife as the weapon in order to protect yourself, it means that you feel as being attacked by something or someone. You know that there is no one that can protect you, but yourself. The knife thats been used against you indicates the dangers you could put yourself into. The dream suggests to be more careful with people you know.
Knife (as a weapon) dream meaning
Read more about dreaming of Knife (as a weapon) in other dream meanings interpretations.