Michael Egnor: "... I oppose the death penalty under all circumstances. ..."
Just to start, what you have truly said is that you oppose any laws under all circumstances. For, all law which commands, "Do this" or "Don't do that", is always at least implicitly backed up by the threat of sudden and violent death at the hands of State agents. Moreover, the death meted out in those circumstances tends to be far less judicious than the capital punishment sometimes handed out by a justly functioning court. I have written about this here, and earlier here, so in this post I'll only touch upon my reasoning. Those interested in understanding are free to read the earlier posts.
To say, "I oppose the death penalty under all circumstances", is to say, "I will not concern myself that my policy preferences would make an ordered civil society impossible." I could put the meaning of this much less charitably.
But, there is an even worse thing you have said -- which is that the criminal, the one who deserves death for, as an example, viciously murdering a member of our society, has the moral right and the legal ability to expel his victim from our society, such that we can hold ourselves to be not duty-bound to seek and apply justice on behalf of his victim. I have written about this here.
To say, "I oppose the death penalty under all circumstances", is to say, "I will not concern myself that my policy preferences would make a just civil society impossible." Likewise, this can be stated in even more stark terms.