I want to walk up to the homeless man and tell him that he smells bad.
That seems mean and dehumanizing. Why would you do that to him?
I want to do it and see how he reacts or if he'll hit me.
Reminding the homeless population about their situation is quite mean. I think you should keep these words to yourself.
It is not that serious. I just want to mess with him a little bit.
It is not serious to you but to him who probably suffers a lot of slights from other people, it is really mean. Even telling a person who is not homeless that they smell can be offensive to them. For a homeless person it is worse.