Reporter: But the bad gurus...
Śrīla Prabhupāda: And what is a "bad" guru?
Reporter: A bad guru just wants some money or some fame.
Śrīla Prabhupāda: Well, if he is bad, how can he become a guru? [Laughter.] How can iron become gold? Actually, a guru cannot be bad, for if someone is bad, he cannot be a guru. You cannot say "bad guru." That is a contradiction. What you have to do is simply try to understand what a genuine guru is. The definition of a genuine guru is that he is simply talking about God—that's all. If he's talking about some other nonsense, then he is not a guru. A guru cannot be bad. There is no question of a bad guru, any more than a red guru or a white guru. Guru means "genuine guru." All we have to know is that the genuine guru is simply talking about God and trying to get people to become God's devotees. If he does this, he is genuine.
Science of Self Realization: What Is a Guru?