Please accept my humble obeisance. All glories to Srila Prabhupada
There was a person in a village who had acquired occult powers through intense penance. He was a close friend of a merchant in the village. Whenever the merchant had any problems or troubles in his business or personal life, he would approach this person and have his issues instantly resolved by virtue of the divine powers acquired through penance.
After all, the merchant was human too. One day, he picked up a quarrel with this divine person over some issue and it became a big fight. Now the merchant was in depression. He lost his self-confidence and fell sick. This was because he was terribly afraid that the same friend of his, who had done a lot of good to him during his heydays, could now deploy his occult powers to harm the merchant, now that he is his deadly enemy. This very thought troubled the merchant and he became fit for nothing owing to depression.
One goes to the temple and prays to the Lord. Imagine that the Lord destroys all his bad Karma. Fickle-minded human that he is, tomorrow he may turn against God and start abusing Him or denying His very existence. Then he would think, Oh! God! when I was close to God, He absolved all my bad karmas. Now that I am no more grateful to Him, will He destroy my good Karmas too? This very thought will make his life a miserable one.
Hence, unless one is a great Bhakta or surrenders to God through a Uttama Guru, the Lord does not interfere with his karmas.