This is a tough one.
Honestly, I think that this would need to be something that would have to be determined on a case-by-case basis. Who determines if the punishment is humane? Shouldn't the criminal have a say in it? Or not?
If it was me, and I knew I had no chance of ever escaping prison or being released, I'd rather just die. I've been binge watching Beyond Scared Straight the last few nights while gaming, and if I've learned anything, it's that the American prison system is hell on earth. If you're stuck inside it and there's no chance to escape, that's a very dreary and hopeless situation. You're a slave-- that's not conjecture, it's literally in the text of the Thirteenth Amendment of the Constitution, and a lot of corporations exploit prison labor, as well. And that's just the things that are internal to your situation, disregarding the likelihood of getting beaten or raped by the other inmates.
I can't speak for anyone else, but I'd rather die than spend the rest of my life in prison with the way that the prison-industrial complex is in the United States.
But there are some people who would probably rather live in prison for the rest of their life rather than dying, for some unfathomable reason. So this isn't the sort of situation that has a one-size-fits-all answer, I think it would need to be left up to the criminal in question, at least partially.
That being said, I am more or less against the death sentence on principle-- I do think that that's inhumane, because it is a permanent solution that erases all possibility for release of the criminal if they're ever found innocent, and it's been proven several times that the justice system is not infallible and that it does make (serious) mistakes more often than it should.