Saul started out humble, but as he gained fame he became arrogant. He started to tweak the commands God gave him to suit his own desires, and eventually, because of his pride, God left him altogether, and drew near to David instead. However, God did not make himself invisible to Saul. Saul knew very well that God was with David and had left him, but he didn't care. He simply wanted to be renowned as a king. He didn't repent of his arrogance, he didn't care to change his ways, and he made empty confessions to seemingly just appease God's anger (and he even stopped doing that after a certain point, and simply paid lip service to God!)
This unrepentant behavior of Saul's progressively ruined him. Even once demonic influences came into play, he looked for other outlets to soothe himself (most notably David and his harp) as opposed to seeing the attacks on him as an opportunity to repent and step back into God's will. He became more and more self-centered, even to the point of attempting repeatedly to kill David in order to restore his fame.
By around chapter 30 or so, Saul kills himself.
Our own unrepentant behavior can seem very subtle. In my case, my behavior involves a long line of lifestyle choices--spending a huge chunk of time online doing nothing but browsing the internet and playing video games. While neither of those things on their own are bad, my heart behind them (especially as of late) has not been good.
I don't even know where to begin with all the implications of Saul's life. You could practically write a book on him (not sure if I just made Bible pun there). The primary point that I want to make however, is that unrepentant behavior is far more hazardous than we may think, no matter how subtle it may seem in small doses... and the cure for it is far simpler than we make it.
When I'm living in unrepentant sin, it seems so complex, as if there were some major math problem I had to solve in order to change my behavior... but friends, it's not complex. Jesus did all the work--all that's left is to repent of our sin and to turn away from the desire to put ourselves above God and our friends.
That's all that's left.