Any criticism of average utilitarianism that claims that it advocates killing unsatisfied people sounds like rigging the speedometer to me. Obviously what an average utilitarian means is "Work to increase people's utility, and measure this by looking at how high average utility is." Drawing bizarre implications like "kill unsatisfied people" from that is obviously an example of confusing the measurement method for the actual goal.

What an ethical theory like average utilitarianism is supposed to do, is give you a description of what constitutes a good state of affairs. Better states of affairs are defined to be those with higher average utility. If killing people increases average utility, and killing people is still wrong, then average utilitarianism is false.

When you're defining something rigorously, "you know what I mean" is not an acceptable answer. This is especially obvious in the case where we're trying to program machines to act ethically, but it's a failing of the theory even if that's not our goal.