एतान्न हन्तुमिच्छामि घ्नतोऽपि मधुसूदन। अपि त्रैलोक्यराज्यस्य हेतोः किं नु महीकृते ॥

etān na hantum ichchhāmi ghnato ’pi madhusūdana api trailokya-rājyasya hetoḥ kiṁ nu mahī-kṛite

These I do not wish to kill, though myself slain, O Madhusûdana, even for the sake of the kingship of the three worlds; how then for earth?

In plain words: Arjuna says he does not want to kill them, even if they kill him. He would not do it even to rule all three worlds, so he will certainly not do it for one kingdom on earth. Madhusudana is a name for Krishna.