In all 3 religions that are believed to be revealed from God to prophets and messengers, i.e. Judaism, Christianity and Islam, the books of all 3 religions clearly states that killing a person in a sin, and in all 3 religions we do not have the right to take a life given by God, not even if that is the life given to us, i.e. our own lives.
Dos God forgive? Sure. But forgiveness has its conditions to be applicable:
1) Regret: a person committed a sin has to regret committing it.
2) He/She has to stop doing that sin (if it is a habit and an ongoing one).
3) He/She has to decide and commit him/herself not to ever do it again.
4) If the sin involves harming another person, like robbery for example, the damage has to be un-done and the money in this example brought back.
If all the above applied, the sin is guaranteed forgives, and it is wiped off the record as never committed in the first place.