Question

I wouldn't say he's the nicest guy around, but I've seen him behave not unkindly before.

Answer 1

If you are telling us to revise the sentence, one error is that there is a double negative. 'not' and 'un' would cancel each other, and the revised sentence 'I wouldn't say he's the nicest guy around, but I've seen him behave kindly before.'

Answer 2

I don't think this is a sentence. the last part doesn't make sense. It should be but I've seen him act unkindly before.