In the Mahabharata war, Krishna adopted many deceitful tactics to overcome Kauravas in the war. The sole purpose was to establish Dharma. But, can Dharma be established through deceitful means? Then, how is it different from adharma?
Clearly, ways of killing Bhishma, Karna, Drona and hitting Duyordhan's thigh are high examples of deceit ways Krishna employed during the war.
This might help:
...Anger that is born of 'I, Me, or Mine' is the one that causes pain and frustration. When you realize that someone is being foolish in his ways and then you get angry (to correct them), then such anger is actually beneficial.
Lord Krishna got angry at Bhishma as well, in the Mahabharata war. Why did he get angry? It was because Bhishma was prolonging the battle each day for no reason whatsoever when he could have ended it. So Lord Krishna broke his vow (to not wield weapons or participate in battle) and took up the Sudarshan Chakra. At that moment, Bhishma folded his hands before the Lord and said, 'O Lord! This is what I was waiting for. If You get angry at me, then my life will be cleansed and benefitted. My life will be perfect and complete if I receive death at Your hands. So your anger too is only a blessing for me'.
Lord Krishna was revered as a Jagatguru (the Guru to the entire world). Both Kauravas and Pandavas regarded Him as a Guru. Bhishma in fact knew this fact for certain. He knew that there was no one greater than Lord Krishna. So he did all this deliberately so that Lord Krishna gets angry, and at least by way of anger, some connection gets established between him and the Lord. After this incident, Bhishma himself gave the clever idea to the Pandavas to defeat him in battle. He said, 'If you make Shikhandi stand before me in battle, I will not take up arms and will lose the battle'.
Excerpt from a Q&A with Sri Sri Ravi Shankar: http://www.artofliving.org/wisdom/the-good-anger?mobile=1
Dharma: that which upholds or sustains the positive order of the world - family, community, nation and the entire universe. Laws may be used to uphold Dharma; but Dharma is not same as Law.
There is no written rule of Dharma or Adharma. It really depends on situation. Krishna's actions were not according to Law; but they were very much according to Dharma (as they were meant to uphold Dharma) and so cannot be called Adharma. You can call them unlawful though.
Please do not intrepet this as "ends justify the means". Ends can ONLY justify the means if the end goal is establishment of Dharma. As an eg: the phrase "Ahimsa Parmo Dharma" is construed as meaning non-violence is ultimate dharma. But the full phrase is "Ahimsa Parmo Dharma, Dharm Hinsa Tathiv Cha"; ie "non-violence is ultimate dharma; so too is violence in the service of Dharma"
