ill-natured; twisted; mean-spirited
Describes a fundamentally nasty or malicious disposition. Often used for someone who is habitually unkind or enjoys being difficult.
That person is ill-natured, so it's better not to get too involved.
ill-natured person; twisted person
Refers to a person with a fundamentally nasty character. Can be used as a label for someone who is habitually mean or malicious.
He's such an ill-natured person that nobody wants to go near him.
根性悪な言い方をしないでください。
Please don't say such mean-spirited things.
性格悪い is a more general, colloquial way to say someone has a bad personality, whereas 根性悪 is a stronger, more specific term for a twisted or malicious nature.
Compound of 根性 (こんじょう, 'disposition, nature') and 悪 (わる, 'bad, evil'). The reading わる is an irregular kun'yomi usage of 悪 in this compound.