If a living person X is painlessly murdered at time T, then this is worse than if the X's parents had simply chosen not to have a child at time T-20, even though both acts would have resulted in X not existing at time T+1.

Example 1:

On a planet Utopia, people live 1000 years in perfect health and happiness. There is no war, starvation, pain, or other things typical on the planet Earth. Just a life full of pleasures, and then a quick and painless death. Mr. and Ms. A decided to have children.

Example 2:

Mr. and Ms. B think that babies are cute, but teenagers are super annoying. They don't care about being alone when they are old; they just want to maximize their pleasures of parenthood. They decided to have a lot of babies, give them a perfect life while they are small, and to kill them painlessly when they start being annoying.

Both examples could be seen as instances of the same problem, namely -- whether it is morally good to create a happy life that must end at some time -- and yet, my feelings about them are very different.