I'm pretty sure we already know for certain that Hitler and Stalin had no desire to believe in Jesus Christ. Remember according to scripture you can only see the father through his son Jesus Christ. As far as Muslims examples are concerned I have no clue what they believe or how they will be in an afterlife. But I do know that God said love the sinner and hate the sin and if somehow a mass school murderer of parkland Florida ask God for forgiveness when he found jailhouse Jesus, then as a Christian who believes that all sins are forgivable if one ask because I believe in the scriptures of the Bible.