In The Tragedy of Othello by William Shakespeare, Lago can best be described by irony. He is widely known by his honesty, but is a lying, cunning, revengeful and heartless man. In the play, there is not much explanation as to why he behaves this way other than how he seems to enjoy (or love) being that way.
Because his never-ending need to plot against other people's lives, there is a lot of parallelism between him and the Devil (but not quite). While he gives many excuses as to why he [Lago] hates Othello, none of them are satisfactory and/or stick long enough to be considered as true.
Therefore, your best answer is a madman.