To make cruel or to treat in a cruel manner; to cause something to become cruel.
From 'cruel' (Latin 'crudelis') with the suffix '-ize' meaning to cause to become or to make, common in English verbs formed in the 16th-17th centuries.
The '-ize' suffix is incredibly productive in English—we can add it to almost any adjective to make a verb, which is why 'cruelize' feels natural even though it's rarely used.
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