What really makes a villain's motivation weak?

I was thinking to myself on villains and their reasons. Even though you spend a lot of effort in their motives on their bad deeds, what can weaken their motivation. Or what makes a villain's motivation weak? Let's say for example, my main villain is Zenkouru and he committed acts of violence and genocide to other nations due to his past and father's advice on the dark magicians origins and being manipulated into believing that the Inochi Valley, the place where he was taken care after his home was attacked by the dark magicians called the Agnarturs, had the dark magician origin stated there, but it wasn't true. So guys, any thoughts.