A dagger in a dream is usually a symbol of aggression. If you stab someone with a dagger in a dream, you could be repressing anger in real life, and you are using your dream to release some of your repressed anger.
The person that you stab with a dagger in your dream can symbolize something in your life that you want to get rid of. For example if that person is tends to do things without thinking, you might want to get of your own impulsive tendencies.
If you dream that someone stabs you with a dagger, your dreaming mind is warning you that something in real life is threatening you. A dream of being stabbed with a dagger can mean that you are feeling vulnerable.
A dagger can symbolize masculine strength.