First, i would like to tell this is basically a spiritual question
but, since you have asked it , i will answer it,whoever you may be, your interest in such matters is really praise worthy, for very few people have this keen interest in such matters of spiritual depth, wonderful ! ! ! ! ! ! !
on seeing himself standing against his own kith and kin , arjuna was couldn't lift his hands to fight his own relatives, he though of leaving the battle ground and even thought of taking "sanyas". it's then when krishna persuaded arjuna to re-lift his arms, he even called arjuna " na-punshak"! ! !
but nothing could persuade him to fight . krishna then explained to him the futility of life,he says, soul is immortal but the body is body is to wear away anuway !  he asks arjuna to be brave and fight ( do his karma alright), without thinking about the results or "fal" , then his shows up to his great form or "viswaroop" to arjuna , here, in the mahabharata, there is a brilliant and beatiful discription given about THE LORD , it,s said "he's neither pure, nor impure, he's both the protector and destroyer, he is everything, yet nothing...." this was to assure arjuna that whatever he does, the lord is with him, and at last he imparts the "gyan" or the true knowledge to arjuna about this world("sansaar") and his duties. he told him how everything in this world is subject to change , except the truth.he asks arjun to be even-minded at both victory and loss, he says that truth alone  remains un-altered , for change is the only truth"
this is the shortest form, in which i could squeeze the entire"gita" into, hope it helps, and keep asking questions like these, i love answering them, thank you dear ! ! ! ! ! ! ! ! !

i love you dear, very few people have such an interest in spiritual matters,God bless you
You are missing the purpose of Krishna's avatar. Krishna didn't just come to Earth to help Arjun win a war and retain a kingdom. His purpose was to teach the world about a new version of "adharma" which was never seen before and which would be prevalent in Kali Yuga, and how to defeat adharma with the weapon of dharma. Siblings killing one another over property is a very common thing in India today. 

If lord Krishna wanted, He would have killed all the 100 Kauravas even before they had tricked the Pandavas to play the game of dice. But if he had done so then the world wouldn't have learnt the lesson which we today learn from the Mahabharat. Lord Krishna also wanted humans to fight against adharma themselves, instead of asking God's help, and that's why he didn't participate in the war, and gave his army of 200,000 to the Kauravs.