If consequentialism were that straightforward that repugnant conclusion might hold. Killing anyone who reaches age 30, though, would diminish the utility of everyone's lives more significantly than the remaining years they'd lose, for they'd also have the disutility of knowing their days were numbered (and someone would have the disutility of knowing they'd have to perform the act of killing others). Also, one's life has utility to others as well as oneself. If everyone were euthanized at age 30, parenthood would have to begin at age 12 for children to be raised for a full 18 years.