Well for starters, a sadist derives pleasure from inflicting pain on others, and in the books it's made abundantly clear that burning people does not please Stannis at all. Quite the contrary, it troubles him immensely. And a messiah complex? Nothing he says or does implies that he believes what Melisandre is selling. He knows she has magic that can be utilised as a weapon, but it's pretty clear that Stannis is straight up an atheist who hates the concept of Gods. He sees himself as the rightful king, which he is in the Baratheon dynasty, and sees it as his obligation as the legal monarch of that dynasty to fulfil his duties as king, which he is doing by fighting against the corrupt region of the Lannisters/Freys/Boltons and avenging the deaths of Robert and Ned.
Really this whole rant is just weird,Marv. I get that you like the show, but this is bordering on ignorance. A lot of these claims seem as though they came from someone who has only had a cursory glance at the books, and not actually looked at the text. Stannis is by no means a peachy clean character, but the material makes it pretty clear that he's got a lot of worth as a human being and as a ruler.