Reddit Karen's are users that get mad, upset, or in general, are disrespectful and/or intolerant of variations with regards to methodology, topics they don't agree with or fit within their definition of a subreddit rules or comments going against their perceived righteousness with respect to what they are commenting on (i.e a post or commenting on a comment from another user.). Engaging with them tends to lead to argumentative responses, name-calling, bullying or harassment via the chat system of Reddit. They will abuse the voting system in bad faith to alter content/comments for personal gain, or to outright stifle discussion, contrary to the intended purpose as stated by Reddit themselves on how the voting system was intended for use. Additionally, they will abuse the "Reddit Cares" messaging system to send you a message for suicide or self-harm to troll you.
"Did you see that rude comment by the Reddit Karen for a post to the Subreddit asking for help?"
"Did you see that Reddit Karen lose it when they were asked for proof of their comment?"
"Just because you don't agree with the facts doesn't make you right. It makes you a Reddit Karen."
"You don't have to act like a Reddit Karen over my observation; I didn't state my claim as a fact, only an opinion."