Bhagavad Gita, Chapter 2, Verse 21

He who knows this to be indestructible, changeless, without birth, and immutable, how can he, O son of Prithâ, slay or cause another to slay?

Once you know something is indestructible, how can you believe you're the one who ends it?

Once you actually see that you're not the one doing the killing here - you're just the instrument - avoiding the fight because "you'd be responsible" stops making sense.