Assalamu Alaikum.My husband uses very bad insults whenever he gets mad. Of all times he's insulted me badly, I actually didn't do anything on purpose to make him angry. He's constantly complaining, criticizing me

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Commenting on what you mentioned, we say that if your husband treats you in this way, then it is against 'living with them in kindness'. Allah Almighty says in the Quran (what means): {And live with them in kindness.} [Quran4:19] At-Tirmidhi narrated that the Prophet said: “The best of you is the one who is best to his wife, and I am the best of you to my wives.”

We advise you to supplicate Allah much to grant your husband kindness and leniency. Choose the right time to advise him in the best way, remind him that the anger is from the devil, and it is the gateway to evil and dissension between the spouses in order to separate them, as Jabir  reported that Allah's Messenger said: “Iblis places his throne upon water; he then sends detachments (for creating dissension); the nearer to him in rank are those who are most notorious in creating dissension. One of them comes and says: “I did so and so.” And he says: “You have done nothing.” Then one amongst them comes and says: “I did not spare so and so until I sowed the seed of discord between a husband and a wife.” Satan (the devil) goes near him and says: “You have done well.”.

Furthermore, strive to beware of everything that could provoke him and result in his anger.
We ask Allah, the Almighty, to grant you and your husband wellness.

Allah knows best.