Let's say we have a man living in a town. He has never done anything sincerely bad. But every day, he dreams about all the horrible things he wants to do to all the people he despises. The only thing that stops him is the fear of getting caught.
Fear of getting caught is an illusion, though. If he truly wants to do it, he will find a way. He doesn't need to wait for the gods of chance to favor his dark predilections. If he truly wants to do those things, he will do them. He will go to a secluded alleyway. He will wear a disguise. He will work where the long arm of the law cannot reach. He will find an outlet for these desires.
Otherwise, he's not really evil, just neutral and spineless. If he's not really *doing* anything, it's all his personal fantasy. And average human beings in the Real World (and, I'd argue, In D&D) have perfectly normal fantasies every day about doing horrible horrible things to the people they despise. It doesn't often get very creative, but the expression "they should be shot," or "They deserve to die," or "Why don't you just drive off a bridge!" are all horrible things that people wish to happen to those that oppose them without being evil.
One day, he happens upon a Hat of Disguise. His inhibition is removed; in disguise he kills his neighbour and rapes the wife; he robs the local merchants; he slaughters the children whose singing annoys him every day.
Yesterday, was he evil? I say yes, deeply. Despite never having acted on his inclinations and outlook - despite no record of evil deeds or actions - he was an evil man. The Hat of Disguise merely allowed him to demonstrate this via deed.
People commit mundane crimes without the intervention of fate every day, simply because they *want* to. A serial killer stalks his prey. A wife-beater comes home at night. A stalker lurks in the bushes. If he's waiting for a Hat of Disguise to fall into his lap, he's just fantasizing. I don't think having that fall into his hands would erase the fear of getting caught enough for your Average Joe (there are ways to see through a disguise) if more mundane ways didn't erase that fear already.
In other words, I don't think your scenario is believable. A person who truly had those deep desires would have no choice but to act on them, in petty and building ways that came about every day. There's never just "one thing" keeping someone from doing something. People are more complex and innovative than that. They can always work around one little problem. Average criminals do every day, both in real life and in D&D. So if he didn't bother to find a way to work around his little fear, he didn't have the drive and dedication enough to become evil, and I find it hard to believe he would suddenly find it with just one element (which has all sorts of problems by itself...a hat of disguise is hardly a writ of carte blanch).
If the tadpoles are evil, they will do things to display that evil, with whatever tools they have available, even if it's only their teeth. Even if the only way they can display their evil is to feed the younger ones to the birds instead of themselves, they will act on it...or they aren't really very evil, if they don't.