Does freezing food kill unwanted bacteria?

Some, but not all. Freezing may kill some of the present forms of bacteria. But for the most part, bacteria may simply freeze the growth state and then continue to grow once food has been thawed. A perfect and well known example of bacteria that cannot be killed through freezing is Salmonella.

To avoid getting sick, your best bet is to properly thaw and cook the meat before you eat it. Like any living population (ex. humans , ants , bees) a natural catastrophe produces a large number of deaths. But the effects are rarely "complete" , meaning that total annihilation is a bit of an impossibility.

The bacteria in your food belong to two criteria -

a) the "wanted" bacteria

b) the "unwanted" bacteria

However they both are bacteria - so the effect of the freezing will be proportionally same on both types. For the bacterial population (in your food) the freezing is a natural catastrophe ! A lot of them may get annihilated because of the severe shear pressures generated by water crystal formation.(Remember ice floats and therefore has more volume than water - for the same mass).

Some bacteria ( unwanted as well as wanted) will survive. That is because their "population" inside is quite huge to begin with. But they will find the going really tough - until the food is thawed ! But once thawed they will multiply quickly to recover populations. In fact this new generation ( according to Charles Darwin) will have more "freeze-hardy" bacteria among them !

Bacteria have been known to grow in extremely hostile conditions elsewhere on earth. Your frozen food hardly nears the "extreme" conditions they are known to survive in.