There’s a far more simple way to say this: stannis wouldn’t let his men burn Asha Greyjoy. That honestly should be enough to convince even the most staunch “d&d are just following grrm’s direction” apologists. The dude wouldn’t let them burn an enemy prisoner, he’s not going to willingly burn his own daughter. He’s not 100% zealous either. He believes because he’s been shown miracles, but he’s still distrustful and hesitant when it comes to the red god. He’s not a holy king on the warpath to punish those who sin, he’s a guy who’s way too dutiful and feels obligated to pursue his duties. His religion is that dutifulness.