Both partners should bear responsibility to some degree. If a partner purposely lies about his/her HIV status, I think that is criminal, particularly if the other partner then contracts it. Criminal negligence. And criminally I think it should be treated as such.
If a partner doesn't know that he is HIV positive and he/she engages in unprotected sex and their partner contracts it, both should be considered negligent in that consensual sex is a two way street, both need to take precautions.
If both partners have protected sex and one is HIV positive and doesn't tell the other, and the other contracts it, he/she IMHO should be held as negligent. To a lesser degree perhaps than the unprotected sex contraction, but maybe not. What I see as negligent there is that the person is not being honest about the risk involved, not whether or not they have protected / unprotected sex. Both are being to some degree negligent if they are having unprotected sex and don't know for certain each person's status.
If they have protected or unprotected sex while not knowing their statuses and one gives HIV to the other, then I see both as equally negligent. The fact that one happens to be positive and the other happens to be negative isn't pertinent to the actions being or not being taken by both parties. At that place and time they are both condoning the same action.
I hope I didn't repeat myself, it's late... And then there's the final category...
People who willingly have unprotected sex with multiple partners of uncertain statuses. Honestly, people blow my mind sometimes. In 2012, in the United States, if you don't know what you could be getting yourself into, then you have my pity. I'm not saying by any means that most people won't be fine, but the chance that someone could kill themselves over an hour's worth of thrill, that really bothers me. It's one thing if you're proven clean. It's another if you're protected. But I'll never understand the thought process in that. Just one of my pet peeves I guess.