This is one of the Gita's most striking declarations. Krishna says: the person whose mind carries no ego, whose intellect remains unstained by attachment — even if that person slays all these warriors, they truly do not slay, and they are not bound by the act.
The statement is addressed directly to Arjuna's predicament. He stands on a battlefield, dreading the act of killing his own kinsmen. Krishna's message is not that killing is trivial. It is that the binding power of any action — even the most extreme — comes from the ego's claim of doership. Remove that claim, and the chain snaps.
This does not mean action has no consequences in the world. It means the inner bondage — the karmic knot that ties the soul to cycles of birth and death — depends entirely on the attitude of the doer. When ego is absent and the intellect stays clean, the soul passes through action the way light passes through glass.