By giving something to someone.

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.

and in this case the owner didn't consent, regardless of the contract's contents

Under US law, I think that's probably true, but a) it's not what I was talking about, and b) I don't think you should be as confident as you seem to be.