Muslims please answer!!!!!?

i am a Christian and i started reading the Koran recently. now in one line (sorry don't remember the reference) it says the god/allah is a merciful god. shortly thereafter (again i don't remember the reference) it says that god/allah has shut the hearts of some against the faith and given others a disease of the faith so that they may not go to paradise. now, how do you reconcile a "merciful" god with a god who purposely sends some people to hell?