Because He can handle it. If we learned anything from the life of David and all of his poetry, it's that God can handle the nasty things you have to say. David was uninhibited in his complaints to God, yet was still considered a 'man after God's own heart". See, God knows what's going on inside you. We all have places of darkness and God wants you to choose to expose them. A beautiful intimacy can take place there.
Whatever your most nasty hatred is towards God, go ahead and let Him know. He's big enough to deal with it and He's the one who can set you right. Trust Him with it.