I often talk about the way in which the God of the Bible clearly structures events such that even evildoers ultimately serve God’s (good) plan. However, even some of the heroes of the Bible were murderers. In particular, Moses, King David, and Paul were instrumental in the killing of people not in self-defense. (Moses killed an Egyptian who had been beating a Hebrew, David gave a military order that was effectively a death sentence, and Paul [at the time named “Saul”] actively hunted down Christians knowing that some of them would be executed.)
I understand that cynics will say this is yet another disgusting aspect of Christianity, that some of its greatest heroes were murderers. But the flip side is that anyone can find redemption in Christ and transform his life. This wouldn’t ring true if the “great men” of the Bible had all had spotless records.
If God can forgive Saul for persecuting Christians and turn him into Paul–the chief theorist of Christianity–then God can use you too, so long as you confess your sins and seek His guidance.