Girl guest: Why is it all right to kill any plants? How are they different from animals?
Tamāla Kṛṣṇa: She is asking, "What is the difference between killing a plant and eating it and killing an animal and eating it?"
Prabhupāda: The same fault. Either you kill animal or plant, the same sin is there. Just like if you kill an uncivilized and if you kill a big man, the punishment is the same—hanging. You cannot say that "I have killed one uncivilized man." No. That you cannot do. Similarly, you cannot kill even plant. But we have to live. Therefore we can kill plant under the order of the Supreme. Just like I have already explained. Kṛṣṇa said, patraṁ puṣpaṁ phalaṁ toyaṁ yo me bhaktyā prayacchati (BG 9.26). Patraṁ means plant. So Kṛṣṇa wants it. So for Kṛṣṇa's sake we can do that. Just like Arjuna did. Arjuna did not like to kill his brothers, but Kṛṣṇa said that "This is My desire." "All right, I shall kill." This is kṛṣṇa-bhakti. When Kṛṣṇa says, we can do everything, not for our personal self.
Therefore in the Bhagavad-gītā it is said, yajña-śiṣṭāśinaḥ santo mucyante sarva-kilbiṣaiḥ (BG 3.13). That another crude example: just like a soldier. When he is fighting by the order of the state, he is getting gold medal and killing. His business is killing. But the same man, when he comes home, if he kills one person, then he is hanged. Why? He could say that "My business is killing. I am soldier. I have killed this man." "No. This is for your account. On the battlefield you killed for the state's account; therefore you were eulogized. You were given reward." Similarly, we can kill only on the order of the Supreme. Otherwise we cannot kill even a plant. Therefore Bhagavad-gītā says, yajña-śiṣṭāśinaḥ santo mucyante sarva-kilbiṣaiḥ. Yajña means for the satisfaction of the Supreme Lord, whatever you do, you are not implicated with sinful activities. And bhuñjate te tv aghaṁ pāpā ye pacanty ātma-kāraṇāt (BG 3.13), and a person who is doing on his own capacity, he is simply acquiring sinful resultant action.