My mother has always been prone to yelling and feeling low in the evenings. This did not usually occur every night. Lately the frequency has increased, and we are at a point where we only have 1 or 2 "good" nights in a week. My father and I have tried to talk to her about it, but this usually results in more verbal abuse. She denies that she is doing this, yelling and shouting back at us that we are intentionally "making her feel bad." In the last week I have tried to set boundaries by trying to calmly talk to her when she starts to lash out at us. This will lead to more defensiveness and scary comments such as "why don't you just kill me if you hate me so much." She will sometimes be contrite the next day, and admit that she feels angry at night. She may apologize and promise to get control of herself. This may happen for a night or two, but she will still make hurtful remarks to me such as "I better go be myself since you seem to think I go insane at night." I am really worried and at my wits end.
Google "sundowning"and see if the description fits some of mom's behaviors.
Has she ever had a screening for dementia?
Consider having her seen by a geriatric psychiatrist or behavioral neurologist.