Karma is that inherent principle of cosmic system that governs movement of life from one manifestation to another, body after body. Karma also acts as accountant general of cosmic system, keeping record of every single incidence of action whether physical or in thoughts.
If karma is the deciding factor then what is the need of god is akin to saying, when we have medicines to cure us what is the use of the doctor.
Everything in cosmic system is created by God, the ultimate master and controller of entire cosmos. God is an entity that is never questionable. Unless grace of God is available, no human being nay living being can survive of its own. Those who did not believe in God believed in some sort of supernatural power.
Call it jala, paani or water, all meant the same.
From the very beginning of life ever since cosmos came into being God Almighty acts as a Dhrishta (onlooker), never interfering with creation of his! The whole cosmic system is directly governed by laws of evolution and karma. As our indulgence, accordingly manifests our destiny. We are solely responsible for our life.
Karma is the record keeper, nothing more! If at any stage of life we suffered, it was owing to karma indulged by us in past or previous lives. We just cannot blame God Almighty for our woes or sufferings, karma indulged by us. We are directly responsible for our fate and destiny. We just cannot blame any other soul atman or God Almighty for the same.