I seem to remember that up until Airplane, he was mostly known for dramatic roles, and was often the villain:
From IMDB[on the part of his career when he played almost exclusively bad guys] The best part is always the heavy. And the meaner and crueler and the worse you are, the more vicious you are as the heavy, the better the hero looks when he whips you. So, the heavy is liable to be a very dramatic, fine acting part. I told my agent at that time, "I want to play heavies who are really vicious and cruel and terrible. I want them to know that they're terrible and I want them to enjoy it."
Not to be pedantic, but the Gladiator movie bit was Peter Graves (who does look a bit like Leslie Nielsen.)