Originally Posted by The_Final_Stand
See also: Episode 4.
"I find your lack of faith disturbing."
Also I don't care whether or not Motti technically outranked him, he was disdainful of Vader's force powers, and a practical demonstration was the best way to bring him back in line.
Correction: It was the best way to bring everyone else
back in line. Motti would've been dead.