Self - interest is another motive. Like Susan Smith, who was having an affair and saw her children as an obstacle to her new relationship. She was so focused on her own desires that she committed the unthinkable act of killing her own children for her own self - serving reasons.
One possible motive could be money. Maybe she was after an inheritance and decided to eliminate those who stood in her way.
Well, revenge might be a motive. Maybe Ruth Finley did something in the past, unknowingly or not, that offended or hurt someone deeply. And that person, nursing a grudge, decides to take revenge by becoming a killer in the story. Another possible motive could be power. If Ruth Finley stands in the way of someone's quest for power, whether it's in a social, business or other context, that person might be driven to kill her to remove the obstacle.
Jealousy might be a motive. For instance, if she felt that another family member was getting more attention or love, she could have lashed out. Another aspect could be mental illness. If she had an undiagnosed or untreated mental disorder, it could have led to violent behavior.
Some serial killer moms may be seeking attention. They might feel neglected in their own lives and think that the only way to get noticed is through these extreme acts. For example, they might want the sympathy of others.
It could also be related to some past secrets or grudges. Perhaps there was an incident in the past that the friends had forgotten about, but the killer held onto it and let it fester until it led to murder. Say, a childhood incident where the killer felt wronged by the group of friends but never spoke up about it.
Personal grudges could play a part as well. For instance, if the yoga teacher had a relationship with someone and then ended it, the scorned person might have become the 'killer' out of anger and a sense of betrayal.
One possible motive could be greed. If there was something valuable among the friends, like money or property, a friend might kill to get it.
Mental illness is a significant factor that could be a motive in a 'killer granny true story'. An undiagnosed or untreated mental disorder might lead her to act violently. Also, fear could be a motive. If she felt threatened in some way, she might resort to killing. In some cases, a sense of self - righteousness, believing she is doing the right thing by getting rid of someone she deems 'bad', could also be a motive.
One possible motive could be jealousy. If one friend is more successful or popular than the other, it might lead to envy and ultimately murder.
As I don't know the real story of this Kasauli serial killer, it's hard to say the motives. But generally, motives for serial killers can include things like power, control, or psychological issues.