The idea that faith can make us unfaithful is really, really scary. Especially when we believe that faith is supposed to be simple - it's just believing in God, right? Just taking Him at His Word? They say that faith is accessible to anyone, that anyone can come to God, but if our faith can make us unfaithful...how are any of us ever supposed to know? How can we ever hope to have real faith?
Believe it or not, this question has been around from the very beginning. It's the question that the faithful have been asking themselves from the start. And it's the entire reason that the Pharisees came to exist.
See, the Pharisees were concerned that they might only understand enough of God to get themselves in trouble. That their understanding might be limited somehow so that even when they thought they were trying to do the right thing, they might not be. That's why (well, it's one reason) that they so painstakingly went through every single command that God had ever given them and tried to figure out what exactly it means. That's why they added thousands of interpretations to the Scriptures. They knew that there would be times they would think they were getting it right only to get it wrong, and they wanted to make sure that they had every little detail right so that they could not possibly mess it up.
What happened to them, of course, is what too often happens to us: they became a people who obeyed God, but didn't really believe in Him any more. That is, they no longer had a faith that could accept a new Word from God; everything they thought they heard, they passed through the fire of their own understanding until they actually created in their lives the very thing they feared - they became the unfaithful faithful. They became the faithless who claimed themselves full of faith.
They had created this very extensive, yet completely narrow and small, understanding of God. If they thought they heard the voice of God telling them to do something (like Saul heard Him say to slaughter everything), they'd go back to their books and start thumbing through their indexes and try to cross-reference and find where God had talked about that and then, they'd follow what they think God means when He says that instead of just doing what God actually said.
They couldn't simply trust Him. They couldn't simply believe what He was telling them. They couldn't just love Him and let themselves be loved by Him.
And this is the problem most of us face, right? We work so hard to become "faithful" persons that what we end up doing is building this gigantic scaffolding around our faith that actually keeps us from believing God. It's keeps us from just trusting Him. It keeps us from loving Him and being loved by Him. It keeps us from actually living by faith and instead, from the most earnest of our hearts, we live faithlessly, all the while claiming to believe.
That's what happened to Saul. It's what happened to the Pharisees. It's what's happening to us.