"Ill nature" refers to a disposition that is nasty, unpleasant, or unkind. If you're trying to describe someone who has this kind of temperament, there are many synonyms that you could use instead. For example, you could say that the person is mean-spirited, malicious, spiteful, or vindictive. Other options include using words like acrimonious, cantankerous, crotchety, irascible, or peevish. Of course, there are plenty of other words that would work depending on the context. Whatever word you choose, it's important to remember that referring to someone as having ill nature may come across as quite negative, so choose your words carefully.