I don't imagine Spirodopoulos as being a thug or even a particularly burly guy. I think when it came to potential physical confrontations, he tended to get things done less on brute strength and more on being a faster, smarter fighter. He would be particularly skilled in judo--using the enemy's strength against them. (Interestingly, if you do research on judo, you'll find out the guy who first started it was quite a bit smaller than Sewell...i.e. about 100 pounds even in his 20's!) Martial arts practitioners who really adopt the philosophy of their art tend to take a different approach to being a "warrior" than a mere "heavy"...and Spirodopoulos takes much more after the former.
(Now, Spirodopoulos probably knows other styles than just
judo, but this would be pretty high up on his list of preferred techniques.)
I don't think that he has to have impulse control issues to be in that job, either. The way you see Sewell in those pics--I really think Spirodopoulos is a decent, easygoing guy off-duty...passionate about things, but not the type who's wild off duty. (He's not at ALL like Starbuck on nuBSG, for instance). And to me, Sewell fits. I've seen certain pics where he CAN get a pretty badass look, but he seems to switch that off when he's just relaxing. That's how Spirodopoulos is...he's all business when the situation calls for it--but he can flip a switch and relax when the time's right.
And I DO imagine some people being surprised when they see him and then find out what he does. He's NOT the stereotype. (And something makes me think he's been a "plainclothes officer" a disproportionate amount of times, exactly for that reason...)
And for that reason I think Sewell fits the bill.