tombowings
Explorer
I could think of a few backstories that would work pretty well for a non-evil warlock. (this is a very simplified example, for the sake of time.)
A man's sister is taken and imprisoned by a devil. The man goes out and find the devil, who tells the man that the only was to get back his sister is for his soul to take her place when he dies. The man agrees out of the goodness of his heart. However, the man was never quite the same afterwards, awful things began to happen around him and people who knew were hurt. So the man left home. After a couple years on the road, he began to get a hold of his dark powers and vows to the local deity to use them to win back his soul from the devil.
A man's sister is taken and imprisoned by a devil. The man goes out and find the devil, who tells the man that the only was to get back his sister is for his soul to take her place when he dies. The man agrees out of the goodness of his heart. However, the man was never quite the same afterwards, awful things began to happen around him and people who knew were hurt. So the man left home. After a couple years on the road, he began to get a hold of his dark powers and vows to the local deity to use them to win back his soul from the devil.