R
Can you define "lifestyle"? What you have said it's not a one time thing because believers can commit murder once and that's ok. How many times can a believer commit murder and it become a "lifestyle"? Twice? Three? Four? Weekly? Monthly?
On the other hand, the person who is falling into it, struggling with it, hating it (but liking it), repenting, then falling again, that person is demonstrating by his resistance that he has some measure of the grace of God in his life, and wants the grace of God in his life. He's learning, he's seeking. Just the fact that he wants to be free is evidence of some measure of God's grace at work in him, before or after salvation, that he has not rejected. God can and will be merciful to that person.