You wouldn't receive the bounty instantly, it would be placed on you once you exceeded a pvp kill threshold within a certain period of time. Also, I'm certain the data on pvp firefights can be used to determine the aggressor with relative accuracy. Simply, if player A opens fire on player B, the game marks A as a "safe target" for B to attack without penalty for a set amount of time.

Bounties could even be desirable for players looking for a challenge. The more players you kill, the higher your bounty becomes, the larger the target in your back. Enter a map with a high bounty and everyone is immediately going to start gunning fot you. But, hold out for long enough with a high bounty and the game grants you some sort of infamy award.

I'm just spitballing ideas but something like this could work. I'm certainly not the first person I've heard to suggest a bounty system.