Yeah, exactly, and I think that aspect of directly showing how Artemis has changed from what he was in the earlier books by pitting him against his younger self is a big part of why this is my favorite book of the series, and why I couldn't help but feel it was perhaps the best.
Haha, yeah, you could go down the route of there not being a choice and predestination, but I agree, I don't think that was ultimately the message that scene was trying to convey. I'm not sure I would go as far as to say that Artemis's lie to Holly shows he won't ever be completely good, but I do think that Artemis's character and values at that particular moment in the series did make the lie almost inevitable, factoring in Artemis's beliefs on strength and weakness, and his mindset of doing what's necessary, no matter how ruthless. However, I think one of the significant aspects of the scene is the contrast in how Artemis reacts to feeling forced to tell the lie to what he felt when he was lying in book one (such as the deception about the sodium pentathol) or even TEC (not telling Holly they'll be cutting off Spiro's thumb until practically the moment of). Earlier on, he would barely feel a twinge of guilt, whereas in TTP, his deep regret show how much he values his friendship with Holly.
I think Artemis still has room to continue to improve as a person from what he was at the point of TTP (I felt like there was an opportunity in TAC, but it wasn't really fully realized), but yeah, if you're defining 'completely good' as a kind of perfect hero who never makes mistakes, I do agree I don't think that will ever be Artemis. I think his particular flaws will be something he will always struggle with, but I admit, I do think he has a chance to overcome them and become a truly completely 'heroic' character, as we would define a hero. (That is—a hero without an asterisk, as Artemis has been through most of the series.) But then, by that definition, none of the other characters could be defined as being completely good either, and those weaknesses they all struggle with are part of what make them interesting and believable as characters. (Holly in particular has some pretty glaring flaws.)
(Oh by the way, have you had a chance to read The Last Guardian yet? I have one last thought, but I'll put this in spoilers in case you haven't.)