Nope sorry this is wrong. Jesus said by their fruit you shall know them. Constantine's actions (murder) tell me he wasn't "saved". What do his actions tell you?
You are presupposing Constantine murdered. You are attacking a strawman. I do not deny that he killed. He was a warrior king. Where is your evidence that he was a murderer? Or are you claiming that all killing is murder, in which case you are an absolute pacificist who does not believe in the potential justification of any killing, including the American war in Afghanistan and the application of the death penalty?
So how did Constantine start his own religion?