I'm going to call my friend an attention whore
Is there a reason you want to call them that?
Not really, but it's the best way I can put him down without breaking up our friendship
Friendships are built around trust and love and caring about one another. I don't see any reason to insult your friend. If you really feel like putting your friend down on a regular basis, it might be best for both of you if the friendship ends so neither of you suffer.
Technically it's not toxic. I'm only trying to help by breaking him down so he won't become an attention whore.
I understand wanting to help your friend by trying to get them to change bad personality traits. However, there are more tactful and gentle ways to go about it. I think it would be best if you politely and calmly talked to them about their behavior and explained how you feel. They are more likely to respond to that than being insulted.