Prayer for forgiveness after murdering someone?

I apologize for using the term "true Christian." I was referring to someone who has been close to God in the past and knows what has been done for him in Jesus Christ, so that in trying to hide his sin he's deliberately running away from what he knows to be true. But not all Christians fully understand that, and some Christian consciences are more delicate than others. I see now that my implying that the term "true Christian" can only be used of someone who is spiritually mature was misleading, off-putting, and not helpful at all.

EDIT: It hits me that I'm the last one who should imply that even someone who knows and accepts all the doctrines of the faith will necessarily live up to them. Keeping this to the writing side of things (please, let's), the Christian female MC of my WIP wants something she really shouldn't have. She does a bang-up job of conveniently forgetting that she shouldn't have it and/or bargaining with God regarding ways it'd be all right to have it after all. And she knows better. The chickens eventually come home to roost, of course . . .

Maybe a better way to put this for @O.M. Hillside is that he should decide what Christian tradition his murderer is from and research what's characteristic of it. Then have him try to get his secret forgiveness in line with that, while avoiding having him do anything that would be antithetical to his conviction. If, say, the guilty party is a 17th century Puritan, he might try to work off his guilt by an extraordinary display of industry and good works. He would not light candles, say Hail Marys, or swear to go on pilgrimage.

 

My personal prayer for forgiveness is usually something like: "Jehovah God, you know why I'm here and you know what I've done. You know my motivations better than I do, so all I'm doing is asking for forgiveness if you think I deserve it. In your son's name, amen."