Krishna sees through Arjuna's hesitation to its root. What looks like compassion — 'I will not fight, I cannot harm my kinsmen' — is actually ego in disguise. It is the ego that says 'I decide,' 'I control,' 'I choose not to.' Krishna calls this resolve false. It will not hold.
Why? Because prakriti — one's inborn nature — is stronger than any surface-level decision. Arjuna is a kshatriya. His entire being is shaped for action, for protection, for standing his ground in moments of crisis. That nature will not stay quiet simply because the mind has announced a different plan.
This is a truth that extends far beyond the battlefield. People often declare they will never do something — and then find themselves doing exactly that, because their deeper tendencies eventually surface. Krishna's point is not that free will is an illusion. His point is that fighting one's own nature with ego is a losing battle.