I think one 'informative' way you can use a negative change arc is to show someone clearly going through a rough time and making bad choices, and in doing so say to the reader "see that? That's what you look like when you do/say these sorts of things".
People are readily willing to sympathize with a character in a downward spiral, and plead with them to stop and turn back, to no avail...but hopefully, after being presented with the perspective of someone in a place like that, they'll recognize and sympathize with the same behaviors in themselves, and in others.
With that said, I don't think a negative change arc has to result in said character becoming a villain. I have a character I've spoken a lot about, who goes through what I think would count as such an arc...the only point of contention being the 'becoming a worse person' part of the definition. If you count being mean to yourself as becoming a worse person, then I guess he does. ^^;
But generally he's the same selfless person he's always been; kind and self-effacing to a fault. He still does what he's supposed to do as the hero of the story; kill the badguy and save the innocents...it's just that he does it by finally embracing the lie he's been fed all throughout the plot: that he is a worthless living weapon, whose only value is his ability to rid the world of evil like himself. He is born around the time the story begins, and he dies when it ends.
And I guess the message I want to send with him is...like, don't let it happen to you, or someone you know. If you felt sorry for this character, you should have; and if you felt angry at the people who encouraged him down that path for their own selfish gain, you should have; and you should realize that, even though this story is fantasy fiction, things like this happen all the time in the real world.
Regardless of whether the arc results in the creation of a villain or not, I think ^that's what makes the best ones. ^^ For the underlying emotion to feel very poignant and real, enough that the reader can come away from the experience feeling like they've gained some insight into human nature. Even if they can't put it into words.