Why is Scorpio convincing?

As others have pointed out, Andrew Robinson's portrayal of Scorpio is so effective because he looks so normal and unassuming. You wouldn't look at the guy twice if you passed him in the street, which is what makes Scorpio's violence - and the glee with which he revels in it - so frightening. Villains - in movies and in real life - don't always come with black hats and mean looks.

I also liked the little ironic touches to the character, like the peace sign on his belt buckle.

Make tea, not war.

He was hired BECAUSE he looked like a choir-boy, and I don't see how the name "Scorpio" is cartoonish in any way. It's a reference to the real-life Zodiac killer.

I didn't care for Robinson in this role. I understand the look they were going for, I just didn't care for his acting. But I watched Robinson's run in Deep Space Nine. He was the best actor on the show. His character was incredible.

Rather elementary or ignorant or naive or closed minded or somethin' like that. You think all psychopathic killers look big, brawny and badass? Or look dangerous? Fookin' 'ell are you not very familiar with or educated in human nature and the fact that literally any human is capable of killing another person without authority to do so? Murderers are all sorts of people, including children.. its possible there's at least one murderer who was actually a choir boy when he was a murderer! I'm sure choir boys have gone on to be murderers! Why do so many people equate being religious to being good anyway? I don't want to bait a religious debate and I don't want to hate but you have to appreciate that millions of people have been murdered in the name of religion.

Ironic you should say, "..choirboy..." In an interview, the director, Don Siegel, told Andy Robinson that hired Robinson because he wanted a villain with a face like choirboy. IMO Robinson turned in one of the greatest villain performances ever. Right up there with Alan Rickman.

Scorpio was a cool sounding name in 1971 and Robinson played that part excellently, he scared the crap out of me when I was a kid. He's a pretty ruthless villain and makes me think of the Clarence Boddicker character from Robocop. He's crazy so he gave himself a crazy sounding name to make the police maybe take him more seriously. I thought he was the perfect actor for that part, he wasn't your normal looking bad guy and he carried himself in a way that was a bit over the top. It's funny you don't approve and yet other people do. I guess that's why they make all sorts of different movies as so many people have different tastes.