Heat kills things because high temperatures triggers chemical reactions which irreversibly destroy some important things. For us humans, this starts happening when our skin reaches around ~60°C. Some microorganisms have found ways to survive even higher temperatures, but ultimately a temperature above 120°C will kill (nearly) all living things, with the exception of some very rare extremophile “archaea” found in thermal springs.
Cold does the opposite. It stops chemical reactions. The way that cold can kill is that ice crystals can grow and expand in ways that pierces cell membrane. That’s why most vegetables will turn to mush if frozen in a household freezer. But bacteria have found a way to survive this: They can turn themselves into a dormant form, where they are almost completely dried out. In this form, these so called spores are not vulnerable to ice formation, so they can survive even extremely low temperatures.