Chapter 2,Verse 30

dehi nityam avadhyoyam
dehe sravasya bharatha
tasmad sarvani butani
na tvma shochitum arhasi

This verse concludes Krishna's lecture from the spiritual point of view.Verses 12 to 30 dealth with "Adyathmic drishti",meaning spiritual angle of Krishna's jist of the lessons of the Gita.
Next,he would next preach from the "dharmic drishti"(ethical angle) as to why Arjuna must fight the Mahabharatha.

Here,he says that you are not the body(deha),but the atma(dehi).The body is only a temporary cloth you are given to wear temporarily.The reason behind giving you this cloth is for you to realize that you are the atma,which is eternal(nitayam)and cannot be slain(avadhyoyam).Hence,you cannot kill Bheeshma or Drona because they too are the atma.The atma is embodied in everyone.Having understood this,you should realize that neither are you the killer nor are they the killed.Therefore(tasmad),you must not grieve(na tvam shoshitum arhasi) over any living being that inhabits this world(sarvani bhutani).

This was from a spiritual angle,which is very subtle,and is difficult to comprehend and believe in.Hence,Krishna would now view the Mahabharatha war from an ethical point of view and speak to Arjuna.There is a general belief that violence is wrong.But just like how,even though steriods are harmful to the body,they are used by doctors for the benifit of the patient,violence becomes necessary at certain points of time in order to restore dharma.Hence,misplaced non-violence is wrong.

An army general protecting the country's borders,when encountering an intrusion,cannot practise non-violence.A surgeon,needing to amputate a person's limb in order to save his life,cannot say that he will not lift a knife.

Hence,violence & non violence are not white and black,and there are grey areas.
This,Krishna will take up from the next verse.