Why does cold kill humans but not bacteria?
Let's look at what happens when a single cell within a cell wall freezes. We end up with a little drop of ice, within which are encased all the cellular structures. Thaw this ice, and all is well - provided neither freezing nor thawing happen too rapidly.
Now take what happens in a human being.
First, the blood in the extremities gets colder, and cools the interior somewhat. But we have mechanisms to conserve heat in the core, at least for a little while. This doesn't help the extremities much. If the blood freezes there, we get frostbite - the tissues die for lack of blood supply. Fingers and toes and the end of the nose, and other extremities, are compromised.
Now let the core temperature drop. Below a certain temperature, conduction in the heart slows and eventually stops. This is fatal.
There are many other effects, all of them related to the relative vulnerability of different tissues to cold. Freezing is generally bad for animal cells, which will rupture because of the mechanical effects of the expansion of water as it forms ice. Frozen body fluids can't do their jobs. Frozen nerves can't conduct impulses, so muscles won't contract once their temperature drops below the shivering point. And so on.