Simple, Wily was jealous of all his (Dr Light's) "accomplishments", and left him in the dark, so.. he gets pissed and turned on him, stealing his robots in the process, and Light's blueprints along with them to make his own. Thus quoting on Wily "I will have finally get ze best of Dr Light!! *RS Wily Laugh*".
Soon on, it developed into a medium-rare rivalry between colleauges on who can outwit who, but doc light's a douche and sends his artificial "son" to attack Wily when he's close to world domination.
Wily realized this, and made Bass/Forte. but due to his faults, he began working on Zero (of course, sending one of his spybots to "borrow" the blueprints to Light's robot X, yet again). However, Zero was the only success Wily ever made.. so THEN he has gotten the best of Dr Light in the Zero series (well, Wily was clever enough to cheat death, watch how everything would play out, with his colleauge out of the way and long forgotten, he would have no problem keeping an eye on Zero).
So ther-- DAMN! PB beat me to it..
;