Why Did Allah Create People Who Would Go To Hell?
It is not to show anything. The point that you are not understanding is that. Allah shows Mercy to the obedient, not the rebellious. Choosing not to create rebellious so they can avoid punishment is an act of mercy. Allah does not allow act of mercy for the rebellious. Acts of mercy are only reserved for the obedient in the afterlife. This different behavior towards the two groups is part of His justice. He will not go against this, because He told us so. This news has thus consequences where prophets would come and warn us because the decision has been made. The news that Allah is choosing to question rebellious, rather than simply not creating them, thus one of the essence of religion is on how to avoid being rebellious.
Because the fetus is innocent and may deserve mercy. The rebellious are not innocent and do not deserve mercy. Allah does not care for the rebellious.