There is an enigmatic passage on the Kaṭha Upaniṣad (1.2.25) which says:

yasya brahma ca kṣātraṁ ca
ubhe bhavataḥ odanaḥ
mṛtyur yasyopasecanaṁ
ka itthā veda yatra saḥ

“There is a person for whom the brāhmaṇas and kṣatriyas are food, and death is the sauce. Who knows where this person lives?”

Who is this mysterious entity that eats even the most powerful Brahmanas and Ksatriyas? The Bṛhad-āraṇyaka Upaniṣad (1.4.6) mentions that “agnir annādaḥ” (fire is the eater), therefore one could consider that fire is the eater mentioned in this passage. However, fire is not a conscious entity, and therefore it can’t perform conscious actions, such as eating. Fire may “eat” in the figurative sense, but not in the direct sense.

One could then propose that the eater must be the living entity, since eating is an action, and jivas perform actions. This idea sounds logical in connection with the analogy of the two birds in the tree, since the jiva is the bird that perform actions, while Paramatma just observes.

In the Vedanta Sutra (1.2.9) however, Vyasadeva gives the proper conclusion: attā carācara-grahaṇāt. The eater is Brahman, the Supreme Person. He is the one who kills and eats everyone, including even the most powerful sages and warriors. How can we understand this?

The point is that God has both a beautiful and a terrible aspect. Srila Prabhupada mentions that those who are devotees see God in His beautiful aspect, as Govinda, while the ones who are atheistic are forced the meet with Him in his terrible aspect as death.

This can be understood in connection with Bhagavad-Gita 11.32, where Krsna says:

kālo ’smi loka-kṣaya-kṛt pravṛddho
lokān samāhartum iha pravṛttaḥ
ṛte ’pi tvāṁ na bhaviṣyanti sarve
ye ’vasthitāḥ praty-anīkeṣu yodhāḥ

“Time I am, the great destroyer of the worlds, and I have come here to destroy all people. With the exception of you [the Pāṇḍavas], all the soldiers here on both sides will be slain.”

The word “kala” means “time”. Time is the force that destroys everything that is material. All material manifestations are created at some point, persist for some time, and are eventually destroyed. Everything that exists inside the universe (and the universe itself) has to face destruction at a certain point. Because the identity we have now is not our eternal identity as souls, we can only keep it for a certain time. Eventually, it will be destroyed. Because kala or time eventually destroys everything. That’s why kala, in the context of this verse can be also translated as “death”.

Time is controlled by the Supreme Personality of Godhead, and thus He is ultimately the eater, the One who destroys everything and kills everyone. This was seen by Arjuna in his vision of the Universal Form. Astonished, he spoke (Bg 11.25-30):

“O Lord of lords, O refuge of the worlds, please be gracious to me. I cannot keep my balance seeing thus Your blazing deathlike faces and awful teeth. In all directions I am bewildered.
All the sons of Dhṛtarāṣṭra, along with their allied kings, and Bhīṣma, Droṇa, Karṇa – and our chief soldiers also – are rushing into Your fearful mouths. And some I see trapped with heads smashed between Your teeth.
As the many waves of the rivers flow into the ocean, so do all these great warriors enter blazing into Your mouths. I see all people rushing full speed into Your mouths, as moths dash to destruction in a blazing fire.
O Viṣṇu, I see You devouring all people from all sides with Your flaming mouths. Covering all the universe with Your effulgence, You are manifest with terrible, scorching rays.”

Why is it so? Why can’t God be just a kind old man who supports whatever we do?

The point is that the material world and temporary and transient by nature. In his original state, the soul has an eternal and transcendental position, but when one desires to lord over this material world, he accepts material identities under the spell of the false ego. Because these identities are different from his original identity as a soul, they can’t be eternal. In this way, we transmigrate through different forms of life, with each body representing a new chapter in our journey in the material ocean.

Once inside a body, we can play for some time. If at the end of this period, we are not ready to return to our original position in the spiritual world, we are forced to abandon it and accept another body, starting again from the beginning. This process is called death, and the Lord is the ultimate controller.

This is ultimately His mercy because if we would be allowed to stay eternally in the same body, we would become entrapped into some kind of eternal torment, without ever being able to return to our original sat-cit-ananda position. Through this process of birth and death in different forms of life, the Lord gradually coaches us into returning to our original consciousness.