Of all the Marauders to play the god of war, I would have first jumped to Padfoot Oldman. He's not Jim Gordon anymore. There's room for him in DC films again.
But on further consideration, this isn't exactly without precedent for David Thewlis. His Professor Lupin was a mild old gentleman who habitually transformed into a demonic force of reckless destruction. Same here. In "Wonder Woman", he's just traded in his pelage for some armour. Both are valid aesthetic choices.
Ares versus Fenris Greyback, lycanthropic Death Eater!
One's a god who's basically a werewolf on the verge of being a Nazi. The other's a werewolf who's basically a Nazi named after a god. It'd be savage, but I'd probably have to give the victory to the actual god.