It's really hard to write a kindly written email saying that someone seems a little dumb or dishonest or an asshole. What am I supposed to say? "Cultural fit"?

It's really hard to write a kindly written email saying that someone seems a little dumb or dishonest or an asshole. What am I supposed to say? "Cultural fit"? If they really are an asshole, they'll interpret that as racism and ageism. I actually do write feedback because of the golden rule. And not one person has thanked me for it, so it feels like it's unwelcome. These emails are also emotionally draining to type. You can actually ask them for feedback after the rejection, or when they give you the pre-rejection feedback form. Many are happy to respond in detail.