The reason why you see so many Sasuke fans is that Sasuke has a large fanbase. There are also a lot of those who hate him. You don't see many Minato fans fighting, because there isn't much hate or love for that character. In your context, every fan is a Karin because most can't help but defend our favorite character, or anything or anyone we're a fan of.
Why do so many Sasuke fans believe in each new ideal he has? Loyalty. Not mindless obsession and lust, like Karin. Also, what Sasuke does since part 1 was always contradictory and leans towards the darkness. Each new thing he thinks of and does is him being him, and fans like Sasuke because he's being Sasuke.
Just because you're a fan of someone, doesn't mean you agree with everything they do. Not mindless obsession like Karin. And even if a Sasuke fan agree with his actions and choices, good or bad, remember that Sasuke is just a goddamn fictional character. It's not fun if you always agree with the good side in fiction. Plus, when you are a fan of something, you're biased. So of course you're more likely to agree with Sasuke if you're a fan.
Thus, your analogy of Karin representing Sasuke fans is off.