Kang might have a point about people dying in the inheritance way, it can't be helped and all participating are prepared for it, but the fact is they made a deal with Yang Kai. And they broke that deal behind Yang Kai's back. Yang Kai himself is a lying bastard, but he's not actually in the business of breaking promises. He just lies about all sorts of things when he's under no obligation to tell the truth or stick to any agreements.