It's Christmas time and this is probably one of the easiest mission. First after waking up head for your door but don't exit yet, instead look at the shelves on the left side and grab the water jug that's sitting on top of it.
Now leave your room and head for the left side of the hallway and enter the last door on the right side. From there you don't need to go down yet, rather than going into the room you always pass when going down, head for the last door at the end of the hallway.
After opening the door turn left and open the door. Go inside the bathroom and take out your water jug. Hover over the faucet and interact with it to fill the jug with water.
Now exit the bathroom and head downstairs to the dining room. Exit the dining room through the same door you used when you stole the rat poison.
Turn left once you get to the garden and take out your water jug. Approach the chairs near the door in the middle of the garden and an arrow will appear. Dump the water in the area the arrow is pointing to.
Now go over to the left side of the door and unplug the Christmas lights. Alistair will then look at what's happening. If you're having difficulty finding the plug, you should just follow the Christmas light cables to the left side.
Wait for Alistair to get near you and you'll receive your target confirmation.
Once he walks over the water puddle he'll fall and hurt his back. Equip your telekinesis and point towards the icicle, Hold the button until the cutscene starts and Alistair dies.