Flower Thrower - Banksy
This is one of the first Banksy's works I've seen and one of the many reasons I like his works.
I agree that the work looks aggressive. I think it's because of a man who holds the flowers. He looks like rebels in some protests throwing granates but instead of them there is those flowers. As you said, they are the only colourful thing in the black and white -painted work and could depict peace and hope. In my mind Flower Thrower critizises how in a cold and chaotic world people try to reach peace with violence.
In my opinion the cropping is precisely right: without the background the message comes out better and the man's pose makes the context so that the message can be understood.