The extremist lives in a bubble, a delusion, that takes what is a fragile and weak ego and creates a fantasy world in which they are important or strong. Thus their Achilles heal is to take apart this twisted and sick alternative reality. First, the idea that others want what they want, or are secretly hiding hate, or would follow them if the other options were removed... this needs debunking. Secondly, other options for building real self esteem and an actually strong ego need to be put into motion, otherwise they will seek the "Bonnie and Clyde" exit strategy. They need to see that even if they were the last asshole standing the "sheeple" (as they see it) still would reject them - but - if they do "xyz" then they can gain true acceptance and join the "herd" instead. Countering extremists is not carrot and stick, it is cold water and a warm towel.