IANAL but if someone leaves their front door open, it's still illegal to walk in and take their possessions. I would imagine this falls under a similar ruling.
If the hacker was entitled to those funds based on the agreement between the concerned parties (implicit OR explicit in the contract) it would not be theft. But it clearly isn't their Ether and the implicit agreement behind the contract stands.
Basically human ethics, morals and the legal system will always trump code.
The comparison to murder doesn't work because you can't consent to murder, but you can consent to theft.
It's not clear to me whether that situation would be taken as consent, but unless you know something I don't, it probably shouldn't be clear to you either.
I mean, legally, what happens is not "consenting to theft". Unlike say assault, which you can consent to, and legally speaking an assault actually happens but consent makes it okay; but with theft, if you consent what happens is not legally theft. (IANAL, but this is my recollection from law A-levels.)
But that distinction is irrelevant here. The point is that there's basically no way for someone to deliberately kill someone and have it not be a crime. But there are ways for someone to take someone else's stuff and have it not be a crime, and one way is if the owner consents.