also: せいあく
na-adjective, noun
ill-natured; bad-tempered; mean-spirited
Describes a person's fundamentally unpleasant or malicious character. Often used in the phrase 性悪説 (せいあくせつ) meaning 'theory of innate human evil', but the reading しょうわる is more common for the standalone adjective/noun.
彼は性悪だから、あまり信用しないほうがいい。
He's ill-natured, so you'd better not trust him too much.
あの子は性悪
That kid is mean-spirited and quickly does nasty things.
Compound of 性 (しょう/せい, 'nature, character') and 悪 (わる/あく, 'bad, evil'). The reading しょうわる is a kun'yomi-based compound, while せいあく is on'yomi and more formal.