A maniac is a crazy person. Screaming like a maniac is never a good way to get your point across.
Long ago, the word maniac was an official psychiatric term that meant "a patient suffering from mania," or manic disorder, involving a euphoric mood and extreme energy. Maniac isn't used in a clinical way anymore, but only informally to mean "lunatic." You might describe yourself running around the house like a maniac looking for your car keys, or dismiss your sports-obsessed brother as "a football maniac." The Greek root is mania, or "madness."