Nice Observation Surya!! In Aadi Parva of Mahabharata there is a description of how duryodhana body has become so strong with Ghandari blessing But you should know what stopped/ made his thighs week? It was Paramatma who know that this being duryodhana will be a adharmic and he(krishna) stopped him from getting complete undefeatable body but his rest of the body was so strong because he was son of gandhari who is was a chaste women (pathivratha).
Lord krishna has asked bheema to attack on his thighs and kill duryodhana. Now lord krishna to accomplish greater dharma, he has used small unfair method to kill dhuryodhana. He want to show to accomplish bigger or greater good one can take a out of law course. So is that mean these people got away from their (may be smaller) adharma? No - For this very deed lord krishna has suffered curse from ghandari - who said all the yadhu vamsha will vanish with clashes between them.
So every event in this universe is interlinked which are linked by the ultimate Paramatma. We can only see a part of that chain and unable to realise the cause and effect of the event.