What is it called when you defame someone and you happen to lose respect for doing that.
"George is verbally attacking John, by doing that George is losing people's respect"
George <blank> himself by attacking John
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George shot himself in the foot by attacking John.
To do or say something that inadvertently undermines one's interests.
You could consider using the verb backfire which means:
(Of a plan or action) have an opposite and undesirable effect to what was intended: 'overzealous publicity backfired on her'.
Your example (You need to change the word order):
George's attack on John backfired on himself.
[Oxford Online Dictionary]
The problem with "slinging mud" or even getting into a "mudfight" with someone is that you'll often end up covered in the same mud. In fact, it's so very likely, that the concept is specifically stated in the definition example...
Casting aspersions with intent to discredit.
The campaign degenerated into mutual mudslinging, each candidate trying to tarnish the other's reputation and looking bad in the process.
There's an expression: "Whoever slings mud, loses ground."
It means that when you demean someone else, you demean yourself as well.
"Mudslinging" means insulting or attacking another person in order to harm his reputation or get him metaphorically dirty. "Losing ground" means falling behind in a competition, or being forced to move backwards. And it's also a play on words (mud/ground) -- when you throw something, you lose some of it.
So in this particular context, one might say "George is slinging mud and losing ground."
George is cutting off his nose to spite his face by attacking John.
"Cutting off the nose to spite the face" is an expression used to describe a needlessly self-destructive over-reaction to a problem: "Don't cut off your nose to spite your face" is a warning against acting out of pique, or against pursuing revenge in a way that would damage oneself more than the object of one's anger.
I think "poetic justice" or "poetic irony" fit the description.
Petard https://en.wikipedia.org/wiki/Petard George made a petard of himself by attacking John.