One can use a simple hack to get round this problem: set any being that has existed but no longer does as having utility zero (for some zero level). In that case, average utilitarians won’t want to bring them into existence, but won’t want to elminate them afterwards.
I’m sympathetic to this line of thought, but setting the number to zero creates unnecessary problems. I think it would make more sense to set the utility of a dead person to be whatever the total amount of utility they experienced over their lifetime was. This has the same result of removing the incentive to kill unhappy people, since (for instance) people who died in their 20s would normally have much lower utility than people who lived to be 80. And it would remove certain counterintuitive results that zeroing produces, such as having someone who was tortured to death end up making the same contribution to the average as someone who died from excessive sex.
I’m sympathetic to this line of thought, but setting the number to zero creates unnecessary problems. I think it would make more sense to set the utility of a dead person to be whatever the total amount of utility they experienced over their lifetime was. This has the same result of removing the incentive to kill unhappy people, since (for instance) people who died in their 20s would normally have much lower utility than people who lived to be 80. And it would remove certain counterintuitive results that zeroing produces, such as having someone who was tortured to death end up making the same contribution to the average as someone who died from excessive sex.