Krishna does not speak in generalities. He names names. Drona — the teacher who taught Arjuna everything he knows about warfare. Bhishma — the grandsire who held Arjuna on his lap as a child. Jayadratha — who will cause the death of Arjuna's own son Abhimanyu. Karna — the rival whose skill matches Arjuna's own. These are not strangers. These are the people Arjuna was most afraid of killing.
And Krishna says: they are already dead. 'Maya hataan' — slain by Me. The past tense is deliberate. From the perspective of Time, the war is already over. What remains is for the events to unfold in the visible world. Arjuna's arrows will be the instrument, but the decision has already been made.
'Ma vyathishthah' — do not be distressed. This is not a cold command. It is reassurance from the deepest possible source. Krishna is saying: the weight you carry — the guilt of killing your teacher, your grandsire, your peers — that weight is not yours to carry. I have already borne it. You need only act.