Too late to edit to add: when I say "continue to develop" I don't mean to continue creating new viable embryos, I mean allowing the existing embryos to mature.

I think both cases are probably unethical (creating to begin with, and secondly, more unethically, to bring them to further maturity, or even to the point of being self-supporting, i.e. fully alive)

If we start going down this road, eventually someone will allow them to be brought to life.

We already know the horrors wrought on other humans by our very selves in the name of "science" in the past (think unit 731, not really science, but "science"), I can only imagine we'd do worse to something not fully human.