Is Leroy from The Bad Seed a psychopath?

I wouldn't judge Leroy to be a psychopath. The obvious psychopath (or sociopath) in the story is, of course, Rhoda, who commits multiple murders without any sense of guilt or regret. The irony of the story is that the seemingly mentally-challenged Leroy is the only one who sees through Rhoda from the beginning; though, at first, even he doesn't believe she has actually killed anyone. It's only when he sees her demonic reaction to his claim that he's recovered the murder-weapon shoes that he realizes she really did murder her classmate Claude.
It's also ironic (though perhaps not surprising) that the self-important, supposedly-learned Monica Breedlove—who claims to know all about psychiatry—is completely oblivious to Rhoda's deceptive nature. Monica is intolerant of Leroy that she judges him, not Rhoda, to be mentally ill, openly telling him so in her angry, pompous way.
Though Leroy isn't a psychopath, he nevertheless behaves in an aberrant manner and does display a vague type of mental illness. His attentions to Rhoda are inappropriate and obsessive. A more "normal" person in his position would probably simply avoid Rhoda instead of taunting her constantly as he does. He's mean-spirited in a childish but sinister way.
From his apparent obsession with Rhoda, it's possible that the movie is indicating that he's a potential (or actual) child molester, but in the 1950s, this subject was off-limits in mainstream films, and there's no direct evidence of it in the story. The most explicit sequence in the film occurs when Leroy's horrifying screams are heard as he's consumed in the fire started by Rhoda.