Why are some relationships so passionate they cause passionate murders of the worst kind? Is it passion or something else?

We have hundreds and thousands of cases where men killed their wives or girlfriends, then the cases were women killed their husbands and cook them(Australia) then we have mothers killing their children when the husband wants divorce. Not everyone should be getting married but society has made it so that anyone can get married and stuff like this happen. Use discernment, don't just marry someone cause they have money or nice things.