You're misunderstanding Stannis's base motivations. He believes he is entitled to be King and that he is the rightful heir, because he is. Tommen is bastard-born and lacks any of the blood that the Baratheon claim was built upon. Stannis is a man of law and justice. In his narrow interpretation of that law, he not only should be king, he must be king.
Do you mean me? No I understand that, that's the very absis of Stannis' character and I would have to be blind not to understand that he
perceives it that way. I however do think that it is is nothing but a giant, smelly heap of bollox.
Stannis can phrase it as nicely as he wants, reciting this claima and that, but bottom line is, he wants power. Who cares if he
wants to be King because he thinks he has the right do. So do half a dozen other people it seems, for various reasons.
He does not need to, he could at any point back down and would likely still get to rule the Baratheon holdings in thw Stormlands.
What I meant was, since Kelthaz seemed to say "Stannis did what he had to do to save Westeros from the White Walkers" Sorry if I misinterpreted that, btw.
No he does not need to be King to do that and Mellisandre knows that, she supports him so that she can stick close to him until the needed moment. And if Stannis reason for all this was really so that he can save Westeros from the Long Night, well then he shouldn't be running around wasting the lives of his Soldiers on costly sieges anyway.
Buttom line is still that Stannis
wants to be King, but doesn't
need to be King.
Along those lines, who does Stannis think will follow him on his Throne now that he butchered his only child?