To make universe stable, there is an equal amount of positive energy and negative energy. There are equal no. of Devtas and Daanavs in the multiverse. Above all these, God exists whose work is to maintain the balance.
But who are we, we are just a neutral substance stuck between positive and negative enegry. Human race is stuck between Devtas and Daanavs. Since we are neutral, God only acts as an observer for us. God holds no privilege for the human race.
Hence, simple law of Karma applies to us. If you do thousand good things and one sin, god has no privilege to waive off your that one sin. You will get the reward of 1000 good things and punishment of one sin seperately.
In other religions, God acts as a judge and can waive off the sins. This is not the case with Snatan Dharma (Hindu Religion). God only acts as an observer for us.This is law of Karma.
Ishwar Bhakti - To get Moksha
Everybody has to undergo sorrow and happiness as per his / her Karma. Then, the question arrises that if God has no role and influence over our Karma, then why should we worship God.
God's worship is important to get free from the mathematical equation of Karma. Ishwar's Bhakti is important to get free from the laws of Karma.
Getting free from laws of Karma is nothing but attaining "Moksha". Moksha is attaining the sixth dimension of multiverse which is known as "Brahma Jyoti". After gaining Moksha, a soul gets away from vicious cycle of birth and death.