Of course, it's just not necessarily the case. I wouldn't even say that it's therefore likely that they aren't and never were a Christian.
Consider that the Lord said we can know a tree by its fruit, and the Apostle John wrote, "you know that no murderer has eternal life abiding in him." Since eternal life abiding in the believer is the primary evidence of belief, it follows that a murderer, at least while committing the act, is not a believer.
We must always consider David when talking about stuff like this. Our Maker Himself says that he is, "a man after mine own heart, which shall fulfil all my will," and that was a quote from deep in Acts (Ac13:22), well after his adultery-murder.
Yes, and when we consider David's sin, we must also consider his prayer (Psalm 51) and that he never sinned again after his repentance. It is written: "David did what was right in the sight of the LORD, and
had not turned aside from anything that He commanded him all the days of his life, except in the case of Uriah the Hittite.
And we should consider John's parents when imagining the idea of David keeping the law for the rest of his life after his repentance. Luke 1:6.
Why doesn't everyone do that? Is it possible to do? Can't an idea we believe or the Spirit we receive cause us to not murder, steal, commit adultery or covet?