What do people gain from killing others?

There are various reasons. According to police statistics, most murderers personally know their victim(s). It's usually some type of revenge or an emotional reaction. Some type of relationship drama.

On the flip side, I've thought about going on shooting sprees like that myself. For a lot of mass shooters, they have certain traits in common. Namely that they're socially isolated.

They may feel like losers, left out, abandoned, angry, unsupported and unloved. They have no one to blame, so their anger goes towards society in general. Many of them don't intend on being caught alive, so they decide to take as many people with them as they can. They externalize their pain.

There's also the factor of publicity. If nobody knows or cares about who they are, then they are going to make headlines to make sure the world knows their name. It's very twisted logic, but from a place a deep pain and shame, it makes sense to them

Well... 🤔

When I was young kids ran around playing Cowboys & Indians or Cops & Robbers with realistic (looking & sounding) Toy Guns. We could say, "I'm gonna kill you!" & "I hope you die!" to friends and have fist fights, watch war movies, cowboy shows with gunfights, and run around with working Pellet & BB Guns killing birds or lizards, etc. Everyone's family had shotguns and rifles easily accessible and regular hardware stores sold guns & bullets & knives, etc.

Shootings, except for big time crime gangs or a rare domestic dispute were unheard of!!
Stuff like that happened in 3rd world countries and far far away.

Then ...

Somebody whined loudly and others got upset and worried, and pretend make-believe violence became "Bad" and all the kids had to hold their feelings in and stifle their differences or they'd be branded as unfit unhealthy and troublemakers by worried ninnys. Soon picking up an "L" shaped stick and saying "bang bang!" got 5 year olds expelled from schools and everyone got trophies, treated like robot idiots and praise for doing nothing special.

You can finish the rest......

A twisted sense of self worth.

Well, I have a VERY FICTIONAL murder fetish, so roleplaying murder is arousing. It’s about total disregard for someone’s life and indulging in only yourself. Also, a lot of fetishes get their kick from being “morally wrong,” so it is that way too. A lust murderer might feel that way.

Some folk also just enjoy it.

Revenge.