It's extremely difficult. Some might turn to religion, seeking forgiveness through prayer and penance. They might believe that by atoning in a religious way, they can find some peace.
It could be due to a sudden fit of rage or jealousy. Maybe there was a fight over something really important like a girl or money. It's a really sad situation.
It's hard to say for sure. The author might have drawn inspiration from real-life experiences but could have embellished or changed details to make the story more engaging.
Some might turn to self - destruction. They could start abusing drugs or alcohol to numb the pain of the guilt. But this is a very negative way to cope.
One possible reason could be a fit of extreme rage or jealousy. Maybe there was a long - standing feud over something like a love interest or a business deal that got out of hand. For example, if they both fell in love with the same person and the competition turned bitter.