Sammo

Stupidity: To quietly kill the hotel valet he is replacing, Diabolik throws a knife at him from distance, hits him in the back, and then drags his body in the middle of the street to the nearest manhole, leaving a long streak of blood he does not conceal. If this sounds pretty risky and unsafe especially for a meticulously planned murder, you have also to consider that in the comic book from 1963 this movie otherwise faithfully adapts, Diabolik did nothing this foolish; he quietly killed the guy strangling him next to the spot he'd dump his body into.

Sammo ★

Plot hole: Diabolik blinks in Morse code to his accomplice the location of one of his hidden stashes. He says it's in 'the third brick to the left' and the street name, but no address or other reference. She (who previously knew nothing about the location) finds it immediately. It's worth noting that the movie is a faithful adaptation of a couple issues of the comic, but this bit of subplot is a fresh addition and the original hiding spot in the 1963 issue was a plausible one (cave behind a specific waterfall).

Sammo ★