An invalidator is any person who is, for whatever reason, prone to being jealous and insecure. In order to think well of themselves, they will say and do whatever they can to harm the target of their jealousies and insecurities, and to discredit them in any way they can. This gives them a false sense of self-worth and superiority. They are all about bringing out the worst in others, and they will use any unscrupulous, malicious, or manipulative tactic in order to achieve this end.
The best ways to confront an invalidator may include simply ignoring them, proving their accusations to be false, or to respond to their negativity with kindness or humor.
Invalidator: "Bob is just useless jagoff. He isn't worth keeping around. We need to get rid of him."
Bob: "I believe you are mistaken, sir."
Invalditor: "Who cares? Go away. No one likes you here. You're a scumbag!"
Bob: "Is there a problem?"
Invalidator: "You're an asshole. We all know it. Get out of here. You're not worth it."
Bob: "I think your attitude is completely rude, sir."
Invalidator: "I'm not being rude. I'm just telling it to you like it is. No one else has the heart to tell you this, but you're a useless asshole. Nothing you say matters. You're not likeable. You're not a person. You are nobody. Know this. Believe it harder than anything else."
Bob: "Whatever you say."