There is a Hadits of Rasulullah from Kitabus Sunnah Ibn Abi Ashim saying that after Allahu Ta'ala created human beings and Jin, He was giving His light, anyone who accept the light with a good and perfect way, they will be those people who will want to accept the guidance from Allahu Ta'ala in their life on earth.
While those who accept it with a wrong way, they will be those who will not want to accept the guidance in their life on earth.
2nd, Rasulullah told in Al Hadits that Allahu Ta'ala already told one of His creation called Qalam (pen) to write whatever happen at that time and whatever that will happen.
Allahu Ta'ala is the one decide whoever become the inhabitants of Hellfire and whoever will become the denizens of Jannah, because it is Allah answers to the question about His Wisdom.
Allahu Ta'ala already know who will become Muslim and who will become Kafir. And He had written beside the Arsy of His "My mercy preceed my anger."
And He had declared TOO "I hath forbid My self from doing Zhalim (oppression) and I doth forbid that to be done by my creation." Source: Al Hadits in Mukhtashar Al 'Uluw of Adz Dzahabi, takhrij by syaikh Nashir Al Albani.
And on another Hadits in Kitabus Sunnah of Ibn Abi Ashim Rahimahullah, Rasulullah said that we won't be able to get out from what Allah has decreed, because Allahu Ta'ala is the Creator of All things and He created the action of His creation. Everything he creates has good and bad, but the creation by Him is all good. Something is bad because it is not connected to Allah, in other word the badness isn't return to Allah, but Allahu Ta'ala created it for Hikmah purposes.
Hikmah is every lesson in this life and everything which for us to realize it that will leads us to correctly worshipping Him. For example: As Sunnah (Al Hadits and Sirah).
So everything will be make easy by doing the action towards it, people who will be the denizens of Jannah will find that doing good deeds are easy, while who will be the denizens of Naar, will find that doing bad deeds are easy. Because Allahu Ta'ala make it easy for people, in accordance to what they keep inside their heart and soul.
Assalamu manit taba'al huda (May peace be unto who follow the guidance).