That is very sad that a culture would expect a woman to remain miserable after her husband dies. Perhaps the expectation is this is meant as a "romantic" courtesy to their lost spouse. More likely the men in the culture believe a woman has lost her purity after she marries, which is an unfair rule that only applies against women. Even women will shame other women for breaking this taboo, because they compete to have their sisters and daughters who are pure to be seen as more desirable. "Til Death Do Us Part" is the vow, but not many people follow it to the letter. Either they take it too far, or they break their vow.

Never heard of the killing sprees you describe in the West as being a result of loneliness. I have heard of people doing some very violent actions against their spouse or loved one because they got dumped or cheated on them. It tends to make some people become very crazy if they feel like they got totally blindsided, fooled, or unfairly taken advantage of.

The more immature and emotionally unstable they are, the more likely they will have a backwards sense of revenge justice, and might take violent measures into their own hands. This can happen in any culture.