Can I purify / kill germs in a water to make it drinkable by putting it in a freezer?
The answer, briefly, is no -- freezing will not make your water safe to drink.

Freezing does damage living cells, mostly because water expands when freezing, and ice crystals can break the cell walls.  Thus, some bacteria will be killed by freezing.  And many larger organisms (e.g., parasites, pests) will often be killed by freezing.  Long-term freezing can increase the number of bacteria killed further.

However, freezing does NOT sterilize food or liquids.  It may decrease the number of bacteria present, and it will generally prevent them from multiplying.  But it is not an acceptable way of purifying water acquired from an unsafe source, because many harmful bacteria will generally still survive.  Also, many microorganisms have spore forms which can survive freezing much more easily and reactivate when thawed.

If your goal is to kill off most microorganisms, boiling may be an easy alternative (and may even require less energy use than freezing).  However, be aware that boiling will generally not affect the levels of any dissolved chemicals that could also render the water unsafe.