In this question, I will make a slippery slope argument. It's probably fallacious, but your task is the identify the point where one step does not inevitablly lead to the next step and explain why not. Here are the steps to the argument:
Skepticism asserts that "knowledge is conviction based on a reason so strong that it can never be shaken by any stronger reason." (Via Descartes.)
All ethical principles are based on convictions that specific actions are either morally good or morally evil.
Different people hold different ethical principles such that one might say a particular action is morally good and another will say that the same action is morally evil. Therefore, each has some reason (however slight) to suspect that their own principle is wrong.
People hold their particular set of ethical principles largely because they learn these principles from outside influences such as parents, culture, religion, friends, etc. Even people who claim to hold absolute ethical principles acknowledge that the reason they hold them is external, generally inaccessible, and accepted on faith by necessity.
Unless skepticism is able to show that a legitimate source of ethical principles exists, we can not know absolutely whether an action is morally good, morally evil, or morally neutral.
Absent the knowledge of morally good and morally evil actions, the best a skeptical ethical system can achieve is some form of moral relativism whether:
d. Or some other scheme based on observation of human behavior.
It seems that Descartes himself halted the slide at point #5:
I recognize that it would be impossible for me to exist with the kind of nature I have — that is, having within me the idea of God — were it not the case that God really existed. By ‘God’ I mean the very being the idea of whom is within me, that is, the possessor of all the perfections which I cannot grasp, but can somehow reach in my thought, who is subject to no defects whatsoever. It is clear enough from this that he cannot be a deceiver, since it is manifest by the natural light that all fraud and deception depend on some defect. (Med. 3, AT 7:51f)