In plain words
Though the form is a human form, the demonic nature covers it, and it is as though soot were smeared on humanness itself.
To proclaim that there is divinity in humanness, the Avatāras take on human form. The principle of the Parabrahman, without attributes and without form, is not understood by everyone; it is not within their reach. God is all-pervading, the Indweller of all, Time itself in form, beyond time; there is fire in a matchstick; it is there without attributes. Give it a form with shape, and it carries out its tasks and works. In the same way, when the formless takes on form, there is the chance to burn to ashes Ajñāna, bad qualities and all the rest, to lead men to contemplate their own svarūpa, and to give them the Ānanda of the Ātma. This very thing is called the establishing of Dharma; it is for this alone that the Avatāra comes with attributes and with form: “Abhyutthānamadharmasya tadātmānaṃ sṛjāmyahaṃ” is the word of the Gītā. Through the Avatāras both of these come about: the destruction of adharma and the establishing of Dharma, the punishment of the wicked and the protection of the good.
But the will to harm people never comes to God, on any day. No, there is none. To teach the message of Prema and root out bad qualities: that alone is His will. When a carbuncle brings on a state in which the body itself would perish, doctors operate and render a great service; in just that way, in the time of Rāma, Kṛṣṇa and the other Avatāras, an operation took place on the battlefield, that is all. When, in the country called the body, the boil called bad qualities grew out of all measure, Lord Śrī Kṛṣṇa, the doctor in form, took Arjuna as His compounder, carried out the war that was the operation itself, destroyed the arrogance and granted health.