You didn't get the presents you wanted this year and that's everybody's problem! Fight your way through a wave of elves, robots and suicide bomber penguins (all the usual Christmas shite) to take your murderous rage out on Santa and ruin Christmas for everyone... spoilt little brat that you are!
ARCADE - Kill Santa, cancel Christmas, get a medal.
SURVIVAL - A never-ending swarm of minions. Drag a load of them to Hell with you for a high score!
ASDW - Move
Arrow Keys - Shoot
Z - Skip, move on, retry etc.
P - Toggle sound
1, 2, 3 keys - Set quality to low, medium or high respectively
You can also shoot with the UIOHKBNM keys if you want diagonal shots to have their own buttons. More for if you want to stick a controller in and assign diagonals to the shoulder buttons or something.