Hubby and I watch a lot of true crime, and yes, it's the significant other a high percentage of the time. I asked Google's AI for typical partner murder motives, and the list is below. (Also, the FBI is outside the door. )

Of course, your fictional character may have had a non-typical reason for killing her spouse. Maybe she's been planning a revenge killing since before they married. Maybe it's a giant conspiracy involving the government. Maybe aliens abducted her and sent a clone back in her place. Maybe her experimental brain surgery went awry. And there's the possibility that it was just a tragic accident that she failed to properly report, and it spiraled out of control.

From Google: motives for partner murders

Motives for murder by a significant other are often rooted in a pattern of coercive control, possessiveness, and unresolved conflict, which can be amplified by factors like substance abuse and financial stress. While each case is unique, research and law enforcement studies reveal several common themes that frequently escalate to intimate partner homicide.

Control and possessiveness

Controlling behavior: A desire to exert complete dominance over a partner is a central motive for many abusers. This can involve controlling finances, isolating the victim from friends and family, and monitoring their every move. Homicide can occur when the perpetrator feels they are losing this control.

Jealousy and infidelity: Sexual possessiveness and jealousy are frequently observed motives. In some cases, the homicide is triggered by a real or perceived affair, or the victim's attempt to engage with others.

Separation or divorce: One of the most dangerous times for a victim is when they attempt to leave a controlling or abusive partner. The abuser, unwilling to accept a loss of control, may turn to violence as a final desperate act. Statistics show that recent separation is a significant risk factor for intimate partner homicide.

Escalation of violence

History of domestic violence: A past pattern of emotional, physical, and sexual abuse is the most significant predictor of intimate partner homicide. These relationships are often characterized by a progressive cycle of intensifying violence.

Threats and stalking: A perpetrator's history of threatening to kill or harm their partner is a major warning sign. Stalking, both physical and through technology, can be a form of escalating control that precedes murder.

Overkill and excessive violence: A high level of rage is often present in intimate partner homicides, sometimes leading to the use of excessive and unnecessary force. This "overkill" is often indicative of the intense emotions involved.

Mental and behavioral factors

Substance abuse: The misuse of drugs or alcohol is frequently associated with intimate partner homicides. Intoxication can impair judgment and lower inhibitions, making a violent outburst more likely.

Mental health issues: While mental illness is not a cause of violence, certain psychological conditions, especially when combined with other risk factors, can contribute. Issues like severe depression, narcissistic personality traits, and suicidal ideation are sometimes present in perpetrators.

Self-defense: In cases where a woman kills her male partner, studies show a high likelihood that the violence was in self-defense after a long history of physical and psychological abuse.

Other significant risk factors

Access to firearms: The presence of a firearm in the home dramatically increases the risk of intimate partner homicide.

Financial disputes: Arguments over money or fear of financial losses associated with divorce can act as a trigger.

"If I can't have you, no one can": Some murders are motivated by a sense of entitlement or a belief that the partner is property. If the victim asserts independence, the perpetrator may feel their only option is to eliminate them.