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Old Karthus was a grumpy old dude who sat on his porch with a shotgun that took potshots at the neighbourhood kids. Also, he was a lich -- it wasn't really a big part of who he is, he just so happened to be one.
New Karthus is a grimdark edgy kid who ran off to the Shadow Isles and offed himself so he could become a lich. Being a dead guy is now his defining character trait and he wants everyone else dead too. Way less interesting, way less fun.
Old Karthus was kind of a dick but if you left him alone you'd be fine. New Karthus is just a big jerk who wants to murder everyone, Saturday morning cartoon villain style.