Is it ethically unsound to accept a graduate admission offer then later withdraw? Note: Similar questions exist on ASE but those pertain to funded programs. 

I have been admitted to an outstanding MS program and waitlisted for admission to another outstanding MS program. Both are unfunded. The first's response deadline is 04/15, and the second said they may not be able to let me know the final decision before then. While the first program is outstanding, I find better fit in second program (mainly research interest, location, and tuition rate). My question is: would it be ethically unsound to accept the first offer prior to the 04/05 deadline, and if I find out thereafter that I am accepted to the second one, decline the first? Since there is no funding involved (as in a Ph.D. program) I would not feel as bad about it.
It depends on what your ethical principles are. For example, if you self-define ethical behavior in terms of whether other people act that way, then it could be. "Accepting" an offer of admission means that you tell the university that you intend to enroll in the program, although if a jet plane crashes into your head and you are hospitalized for 5 years, you may be unable to make good on your word, due to unforeseen events.

In your case, it would be dishonest to accept and then decline, because the acceptance would be a lie (you don't intend to pursue the MS, you intend to pursue the MS if there isn't a better alternative, and this fact isn't unknown to you). An honest acceptance would be a conditional one, telling them that you provisionally accept subject to a change, owing to getting the second offer. That then allows the first institution to withdraw their offer, or to accept your conditions. It also allows them to extend an offer to another candidate, whom they might not be able to make an offer to (in time) if you accept under false pretenses.