Karna accepts the nature of all humans/ex-humans. So long as you don't give him a reason to stab your face off, you'll be alright. Hell, at one point in CCC he even says he'd give Hakunon + Servant a warm reception of tea and cakes if he had the means to procure such things.
That said, with what's about to happen, I recommend keeping a distance of at least 50, no, 100 miles.