The concept of mercy in and of itself is not subjective; there are merely situations where moral conflicts arise. The situation which you described is complicated but what about this one: If god gave us all free-will and different levels of intellect, encouraged us to use them to seek the truth and some people genuinely believed Islam was not the truth; are they really worthy of punishment? There is no way that all of us will come to the same conclusions. If you claim that it is so obvious that god exists and Islam is the one true faith; is it not god's fault for creating them with a deficiency to their intellect? The position of religious people when it comes to those who disbelieve in their religion is that they are either too arrogant or ignorant, which is arrogant itself. Your beliefs are unsubstantiated and cannot be proven yet you tell people who seek answers to "humble" themselves. I don't reject Islam solely on the basis of science, I reject it because it's concept of god and it's nature of revelation are unconvincing.What is considered merciful varies from person to person. For example, one may consider it merciful to just go ahead and pull the plug on a dying grandparent and another may consider it the most despicable action known to man. Everyone person's thought process and consequentially moral ethics varies. This is a good comparison for the idea of mercy within Islam. The fact that you were born into Islam and raised in a Muslim environment may be considered a great mercy in itself. You were blessed by Allah (swt) but you've been confused in this Western and secular environment and I genuinely hope you find the path back to Allah (swt). Just always keep in mind why you exist and always remember to keep yourself humble. Because the "science" you place all of your faith in is constantly changing. The Holy Qu'ran has always remained the same. There's this new wicked idea that exists in secular nations that men have become their own gods and that's a recipe ripe for diaster.