Instead of naming something concrete, it makes more sense to define what the only legitimate basis for morality and the human good is, which is human nature. If you deny that, then there is indeed no possible objective basis for the good. You could not differentiate between any two human action. Decisions would be entirely arbitrary. It would make no difference what you did, except factually in the sense that you did one thing and not another.
If you observe any animal or living thing, you will generally see it behaving in ways that seek to actualize it as the kind of thing it is. The nature of a thing bounds the potentials it has, and so circumscribes the limits of what can be actualized; this is a basic feature of all things, living or not, that they are "causally composed", as it were. In any case, this activity is not necessarily conscious. No squirrel is thinking "Gee, I need to collect nuts to grow and nourish my body and avoid predators so that I can produce offspring and actualize X, Y, and Z." In such cases, the squirrel is moved by various inclinations and appetites whose proper satisfaction actualizes certain ends of "squirrelness". A good squirrel (not in the moral sense, but in the sense of it exemplifying squirrel nature) is one that is able to actualize these potentials and does so to realize its squirrel nature. A bad specimen is one that cannot or does not. So, if you get a squirrel addicted to meth, and all it does is do things that get it more hits of meth while neglecting or impeding the realization of its squirrel nature, then you have a failure or deviance opposed to the good of the squirrel. The same could be said of a squirrel that is lethargic or one that lacks limbs.
Human beings are no different in this general sense, save that human beings are able to a) comprehend their circumstances, at least somewhat, and b) choose between apprehended alternatives. This means human beings are moral agents. So, here, a human being bears a certain responsibility for his choices and actions. If he chooses to act against his nature, especially as a rational, moral, and social agent, then he is acting against his nature and thus against his good. And if he is acting in such a way while understanding that he is doing so, then he now also has moral culpability for his defective actions.
In short, to be the kind of thing you are by nature is what is good. The act in accord with your nature is what makes good actions. Death is not good per se, and to act to destroy yourself is opposed to your being human and thus to your good. To intentionally do so is morally evil. (This must distinguished from self-sacrifice for another, which can be in accord with human nature under certain circumstances, but it is not the case here with Kahneman.)