As long as you use soap, the water temp doesn't matter. In fact, you cannot get the water hot enough to make a difference and not burn your hands. Using hot water is just a waste of gas (if that's the kind of water heater you have) and water (if you are letting the water run in order to let it get hot).
That's not why. The soap loosens the bond between the germs and dirt and your skin. The germs are still alive but are now suspended in the soap where the water can now wash them away. The more agitation, the better the cleaning. Killing germs by the heat caused by friction has nothing to do with washing your hands, especially when most germs will only die when exposed to just under boiling temperatures for at least 5 minutes.
http://scienceblog.com/10882/hand-hygiene-truths-myths-and-misinformation/
http://theconversation.com/mondays-medical-myth-you-have-to-wash-with-hot-water-to-kill-bugs-3086
(Edited because one of the links didn't work.)