Many of the reviews of this movie definitely don't understand it.
It's a mix of insecurities, depression, love, obsession, and mental illness. These two both have emotional issues, that somehow work in their bizarre relationship.
The wife clearly has commitment issues and needs to feel that excitement she craves, since she doesn't find it anymore in her marriage.
The husband also clearly loves his wife, and believes she is his other half in life. So much so that he puts up with her infidelity over and over. But everyone has a breaking point so he thinks killing those men will stop her.
In a way it does, once she is 100% sure he is killing them. It satisfies her sadness with that excitement she craves, and now sees her husband for how much he does love her.
I loved watching this movie and seeing real life mental illness and emotion between partners.