Kill anyone who opposes you with a knife in your hand and a silly grin on your face. You can also collect new weapons from around the map, or buy them. But real men use knives.
WASD or arrow keys move, and mouse attacks. Also, dash with space, and use your item (such as a grenade) with right click.
If it looks like someone's following you, toss a grenade behind you and see if you can catch them unaware.
If that doesn't work, put that knife and silly grin to use and gut them like a pig.
This is Massacre after all.