Arjuna cannot understand how Krsna can advise him to fight Bhisma and Drona who are worthy of worship. He addresses Him as Madhusudana (killer of Madhu) and Arisudana (destroyer of enemies).
The quality of action is gauged by the intention behind it, not the action per se. The same action could be sacrificial or bestial depending on the motive driving it. Besides, Arjuna knew he would have to fight Bhisma and Drona. When his intellect was clear he accepted it. He has now allowed emotion to cloud his judgement hence the problem. In fact, just before the battle began Yudhisthira had appealed to Bhisma and Drona to come over to their side or, at least, remain neutral. They said they were mere subjects beholden to the Kauravas and had no choice but to fight for them.
Arjuna believes his problem stems from an external cause. It is Arjuna’s passivity and confusion that is the cause. Krsna addresses this and empowers him from within. He is then able to face the challenge and win the battle. The moment you attribute your problem to something outside you will never find a solution to it. The truth is there are no problems outside. All problems are internal. So look within. Strengthen your intellect. Empower yourself with powerful positive values. The world will cease to trouble you.