I can think of lots of reasons why shotgunning your way through some 2D pixels might turn your day around, but whatever the cause I recommend Butcher. It's a three-level prototype of an ultraviolent run-and-gun platformer from the makers of Soldat and King Arthur's Gold. You can play it in your browser now and it is distracting me from the neighbours drilling, sawing, sanding and hammering their way through their walls and my patience.
Here's a trailer:
I was able to complete the three levels in my first attempt, but I've gone back a few times since to tinker around with different weapons. Aside from the hearty shotgun, there's also grenade and rocket launchers, a railgun, a pistol, a rifle of some sort, and more. Also there's a kick button, for when you need to silence the wails of your fallen enemies. It's pretty gross! Expect gibs, pixel blood dribbling everywhere, and I'm pretty sure at least one instance of people becoming intestinally stuck to a deathly sawblade.
Soldat was a similarly satisfying (though less gory) 2D shooty game that took equal inspiration from Counter-Strike and Worms and which was released as shareware in 2002, while King Arthur's Gold took the team-based 2D multiplayer and made it about castle-building, digging tunnels and launching catapults. The team appear to have a few other new projects on the go at present, too.
Play Butcher in your browser here.