I think you're a bit off in that assessment, during the fight with the 'children' he and his two companions killed at least a hundred men - heavily armoured by the end - with little more than swords and clubs. His strengths seem to lie in fighting one-on-one but still it'd take a lot of human men to stop him. Also the Forkrul Assail 'fight' I think shows how tough the FA are/will be if we get to see more of them.
Karsa is Thelomen Toblakai meaning much bigger and stronger than the average human with a far greater pain threshold and recuperative abilities. They consume blood-oil which seems to be made partly of otataral, thus negating lower-level magic and even altering the effects of High level magic. Add to this an unbreakable sword which through the souls chained to it, appears to be a warren in it's own right. He chased off his seven 'gods', killing one and became the Knight of Chains. He has since killed
two hounds of _Darkness_.
As a character he's definitely over-powered and I think his fight with Icky proves that, but there has been some progression to his character so I don't see it as being an instant drastic change.