I’m sure Moses wondered if God could use him since he had killed a man, but God used him to deliver the Hebrews to freedom. David led the armies of Israel on many campaigns which is why God wouldn’t let him build the Temple. It was because of all the blood that he had shed while he was king. There have been many people who have done many things in their lives that we might say are too bad for God to use or to forgive. God can even use pagans to do His work when He chooses to. Remember this:
The Living Message of Christ 