What do you think of me telling a coworker that people have noticed he's flakey and antisocial?
Why are you going to tell him that?
Because I'm selfish and want him to be sacked.
I'm not sure how that would get him fired. Either way, if you must tell him, you should tell him in a way that is kind and not insulting.
Why should I be kind?
You should always try being kind to people, because how would you feel if someone randomly said terrible things to you? Treat others how you would want to be treated.