Dreams involving violence, particularly those where you feel the need to defend someone, can often reflect inner conflicts, fears, or feelings of powerlessness in waking life. In your dream, driving your mom's truck may symbolize a sense of responsibility or a connection to your family. Stopping at your friend's house suggests that you are concerned about your friend's well-being.

The abusive situation you witnessed could represent your awareness of real-life struggles, whether they pertain to your friends or to situations where you feel unable to help. Standing up for your friend indicates a strong sense of loyalty and protectiveness, but it also brings you face-to-face with aggression, which can symbolize your own fears or anxieties about conflict.

The act of shooting may symbolize a desperate attempt to regain control or to eliminate a threat. The fear of going to jail afterward reflects feelings of guilt, anxiety, or concerns about the consequences of your actions, whether real or imagined.

Overall, this dream might be highlighting your fears about violence and conflict, your protective instincts towards loved ones, and perhaps feelings of helplessness when faced with difficult situations. It may be beneficial to reflect on any real-life situations where you feel a lack of control or concern for someone's safety and consider how you can address these feelings constructively in your waking life.