The main component of the drug is explicitly identified as having produced madness in human subjects, which Griffin doesn't know when he starts taking it. "The drugs seemed to light up my brain," he tells his solicitous GF, later on, then goes off on a rant about how the moon and the whole world are frightened of him, frightened to death! So yeah, I think this is a weaselly way to keep movie-Griffin "sympathetic", ie he didn't know any better! Personally, I'm fairly certain this arrogant prick-hood and paranoiac fantasy always nested inside him, but whether it would've flourished without the drug and the invisibility is debatable.