Sam, I think that’s taking a very simplistic reading of shaul. You think Shaul, the Mshiach Hashem, the nechba b’keilim, just blatantly disregarded a tzivui?
Presumably he thought he was following the tzivui, after filtering it through what he thought Hashem wanted.
So where did he go wrong?
Chazal tell us: he was rachman bmkom achzar.