Is cannibalism while stranded in life boat ok? No defense. It's murder

The "no defense" proposition isn't entirely accurate. This particular court in 1884 said that necessity was not a defense to murder. A FL, GA, SC, NC, etc. court in 2009 may disagree.

A defendant could also assert insanity or diminished mental capacity as defenses. Today necessity can be a defense to murder in some jurisdictions and some facts. So it is not precluded per se as this decision says. I have no idea whether international law could be controlling here or what that international law is.

Of course all of this is semi-moot because US criminal law is dominated by statute, not common law.

Even if the crime carries an automatic death sentence, it's still worth the risk.

It isn't too hard to hide the body. . . .

This is an 1884 case law.