HELL! Why does the freezing of water, exothermic process, become spontaneous at temperatures below 0° C even though the entropy of liquid water decreases during the phase change?

Because the melting of ice as a spontaneous process (or reaction).

Explanation:

Here is an example:

Take a ice on a plate and leave it for half an hour (conditions). After half an hour, we notice water instead of ice.

This process took place without any intervention from your side. Heat absorbed by the ice from surrounding is due to set of conditions not due to your intervention.

So, we can consider melting of ice as a spontaneous process (or reaction).