Hi Kanaka,
You are right that Krishna did not play fair. But then 'All is fair in love and war' is the saying. Besides, Krishna was a God, unlike the others. He could do as he pleased!
As you said, Karna was a great soul and a righteous warrior. He did not deserve the fate that was dealt out to him in the Mahabharata. That is what makes the Mahabharata such a realistic epic, because good and evil are not clearly separated. Kunti who should have suffered got away in the end!
The sad realization is that not everything is fair in life!
Regards,
Sowmya