I don't know how many times I've read 1 Samuel but several. No matter how many times I read the Bible I find new things. Now some of that is just me because I can read most any book and forget it pretty quickly. Hopefully some of it is spiritual growth or I like to think so anyway.
The first thing that Saul did that displeased God was not waiting on Samuel to offer the sacrifice prior to battle. The account relates Saul's reason for doing it. His soldiers were abandoning him the longer they had to wait. So Saul decided he should go ahead himself and get things moving before everyone deserted him. He didn't have that many soldiers in the first place and he was facing a much larger force. I write all that just to indicate that I can understand his thinking.
At first I thought that Saul offering the sacrifice didn't seem like as big a deal to me as it apparently did to God. That really puzzled me and I have to think about things that puzzle me.
I think the thing is that if you really know that God is on your side then you do not require an army greater than one. In other words God plus one is about the same as God plus a thousand or God plus a million or even God plus a quadrillion.
So I've concluded that Saul did not really see God for Who He is because Saul thought he needed a human army and as big as he could get.
That view of God has to come from the inside -- the heart we usually say. It's one thing to have thoughts and theories about God. It is entirely another thing to "know" God emotionally, viscerally.
Another thing is that Saul apparently thought God did not keep His word. It does not take much Scripture reading to realize that God takes His Word very seriously.
What Saul did was extremely disrespectful to God.