Stannis doesn't think that Tommen is his nephew though, nor is he.
He would definitely be light on the pardons and heavy on the burnings if he ever takes the capital.
That's why I don't think Gurm will ever give him the throne, let alone the final king of the series. If that were the case, he'd kill Cersei, Jaime, Tyrion and Tommen Lannister, Daenerys Targaryen, Barristan Selmy, Jorah Mormont, Victarion and Euron Greyjoy, Roose and Ramsay Bolton, Arnolf Karstark, Aegon Targaryen/Blackfyre, Jon Connington, Mace and Loras Tyrell, Randyll Tarly, Paxter Redwyne, Brienne of Tarth, Walder Frey (and probably other Freys), Doran Martell, et al.
So what are everyone's theories on who the "younger brother" will be that (allegedly) kills Cersei?
Jaime?
The Hound?
Stannis?
Tommen?
Tyrion. Jaime becoming a kin-queen-kingslayer would be interesting, though.