Darth Maul is a villain, plain and simple, and every aspect of this guy's character shouts "guy you don't want to owe money to."
Let's start with the name. The word "maul" means to be injured by way of a thorough beating. It's what lions do to poor little antelopes. It's not pleasant sounding and is a far more intimidating name than, say, Darth Giggles or Darth Tenderheart.
Then there's his appearance. We know you shouldn't judge a book by its cover… but just look at this guy. He dresses all in black, and his face sports horns paired with demonic markings of black and red, making him look like the offspring of Satan and a rabid Atlanta Falcons fan.
Darth Maul is also the strong silent type. He has three lines of dialogue in the entire film, but those few words key us in to how malicious he is:
"At last we will reveal ourselves to the Jedi. At last we will have revenge."
It's only his second line in the film and he's already talking about revenge. Even Iago had more to say before getting into his "I hate the Moor" spiel.
Unfortunately for the Jedi, Maul has the skillz to back up his revenge talk. He is able to hold his own against both Obi-Wan Kenobi and Qui-Gon Jinn in a lightsaber duel. When fighting Qui-Gon alone, he slays the Jedi right in front of Obi-Wan. He almost kills Obi-Wan, too, but Obi-Wan manages to outsmart and kill Darth Maul instead.
And, true to his silent nature, Darth Maul takes his secrets with him to the grave.