^God is the Most Merciful and Compassionate, and
He is also severe in punishment, and also Just. He is not Merciful in such a way that would be unjust (such as by allowing evil people to enter His Paradise). He is also most Wise and we should never question what we know to have come from Him.
We were created for a reason- to worship God alone. I am not sure if you know this, but before Adam was placed in earth all of humanity was drawn out from him and stood before God, and we accepted the 'challenge' to have freedom of choice and live our lives on Earth and accept God's messengers. So, this is something we agreed to, it isn't something that was forced on to us, we choose it.
You might not make much of rejecting God- but it is the height of
arrogance and pretence of self-sufficiency, and only God is self-sufficient.
We might not understand the enormity of the crime of rejecting God, or associating partners with Him, but is only because of our own ignorance. For example, Allah swt mentioned in the Quran, that the act of associating partners with God (more specifically, the claim that He has a son) is so terrible that:
Whereby the heavens are almost torn, and the earth is split asunder, and the mountains fall in ruins,
[19:90]
...and yet mankind just can't understand.
God is the one who makes the rules, not us. He is most worthy of praise.
Oh, and you also for get that rejecting God isn't only about not believing in Him- you also reject to do as He commanded you to do, having the audacity to disobey Him, the arrogance to refuse that He even exists, even though He is the one who created you and nourished you and gave you so much blessings and signs to contemplate, you come up with cheap arguments such as not believing that God would allow such injustice to happen in the Earth, thereby considering Him to be unjust and unaware of what He has created and not able to set things aright, although He has said:
Consider not that Allâh is unaware of that which the Zâlimûn (polytheists, wrong-doers, etc.) do, but He gives them respite up to a Day when the eyes will stare in horror.
[14:42]
...you deny God His rights, and also you deny mankind their God-given rights by refusing God's plan and creating your own cheap human version, thereby causing trouble and corruption in earth by following a flawed governing system... etc