Yeah, its kool you understand the idea that God rewards people for their good. So if we're to say that God will reward someone for the good that they do to please Him, thats fair right? And the people who don't do good to please Him, then why should they be rewarded by Him? That's perfectly logical right?
Now for us to understand what is good and bad in the sight of God, He needs to send us Messengers' to clear this out for us. He could have shown Himself clearly - but then there wouldn't really be a point of the test since the answers would already be clearly given to us. These Messengers are humans like us, and they convey the message clearly and don't ask their reward off the people, they expect their reward from God. They tell people the clear cut way to differentiate between the good and evil.
The problem lies here; If someone doesn't accept God's Messenger, then their rejecting God's Message. What does that mean? It means that their saying that God's lying right? Do you think that God will be pleased with that?
Especially when the message makes pure sense, i'm not talking about the secondary issues here [in regard to the practises of dressing, eating and all that] but more in regard to the beliefs i.e. The Creator & Sustainer who is to be obeyed, the messengers, the concept of this life being a test and being rewarded or punished for our own deeds and actions.
We've also explained the concept of being punished in the hellfire. Even christians themselves believe that those who dont' "accept Jesus son of Mary as their saviour" will be in the hellfire forever. We know that Jesus son of Mary was a servant and honorable Messenger of God, we know that no soul bears the burden of another, and we are responsible for our own deeds and actions. We also said that the reason why the disbelievers may be in hellfire for eternity is due to the fact that once the message is given clearly to them - if they reject it, they do so out of arrogance. And if they were given a life of eternity - they would remain in that state of disbelief forever.
What was the main message the Prophets called to? To the worship of our Creator and Sustainer Alone, to be dutiful to ones parents, to enjoin the ties of kinship, to establish the prayer, to help the needy, to enjoin the good and forbid the evil, to perfect ones manners etc. There is nothing evil about that, infact this is what people really want in life.
"How can you deny God, when you were dead and God gave you life? Then God will cause you to die, and then revive you, and then you will be returned to God." (Quran 2:28)
And among His Signs (in this), that you see the earth barren, but when We send down water (rain) to it, it is stirred to life and growth (of vegetations).
Verily, He Who gives it life, surely, (He) is Able to give life to the dead (on the Day of Resurrection). Indeed! He is Able to do all things.
[Qur'an 41:39]