I am looking for a right answer, why did the Akashvani said that Kamsa was going to be killed by the 8th son of Devaki and Vasudev. He loved his sister so deeply that he himself drove the chariot. He married Devaki with such a pompous way that anyone could imagine, Well to say. None of the text reveal that kamsa was evil and cruel before akashvani. He used to go out for wars every now and then, that was his ambition but there is nothing I read about reason that made such a horrifying announcement by Akashvaani?
Kamsa was an asura named Kalanemi in his previous birth (see Bhagavatam 10.1.28 here or 10.51.41 here). So his loving demeanor was highly superficial - once his asuric pride was offended he manifested his real nature and didn't even hesitate to attempt murdering his beloved sister on the day of her marriage. Great Bhagavata commentator Vishvanath Chakravarti answers your question in his commentary on 10.1.34. The summary of his explanations is that one of the goals of Krishna avatara was to kill Kamsa (Kalanemi), but the devas started worrying that since Kamsa is so affectionate to Devaki, Krishna may not wish to kill him without any cause, so they announced the message to Kamsa to reveal his true nature. But this akash-vani they also conveyed to Vasudeva and Devaki fearlessness by letting them know that the Lord will soon appear as their son.