While the murders that Manson committed or had committed were truly terrible events, the truth of the matter is that, if he had sincerely and truly turned to Christ for forgiveness God would forgive him. That said, we don't know what was on his heart prior to him taking his last breath. If he died without Christ, then we know that he only has condemning judgment in the lake of fire to look forward to and he now knows the truth regarding the existence of God.
I would make mention of the fact that those people who have not received Christ and yet have never committed such acts of violence and murder as Manson did are also on their way to condemning judgment. For scripture states that "there is none righteous, no not one. All have sinned and all fall short of God's righteous requirements." Since believing in Jesus is the only way to be reconciled to God, then everyone who hasn't received Christ are headed for condemning judgment.