Those situations are certainly understandable and forgivable.
But there are terrible people out there who for one reason or another desire to control and manipulate their partner. They may be sick but it does not excuse or justify continuous abusive behavior.
Family member’s wife started experiencing gradually pronounced schizophrenia over a few years and she became abusive to her husband until it ended with a short stay in a mental institute and then medication. She still wonders who she was at that time…
Some people suffer diabetes as they age, and more than you would guess start experiencing mental breakdown that can manifest as abuse to a partner.