To take it a step further, I always understood the maiseh with shaul in a similar vein with what I wrote in this post. When a person doesn’t act according to the Torah, it is bad even if currently it is a “chumrah”, because once you’re out of the Torah’s morality, you’re on your own morality, and it is ??? ???? ??????.
Which is why the gemara says that he was told “al tehi tzadik harbeh” – and then contrasts that with when he killed Nov ir hakohanim, and was told “al tirsha harbeh”.