History holds many records of callous rulers.
冷酷 also means 'cruel' with a nuance of coldness, but is more common than 酷薄 and can be used in wider contexts.
Standard kanji spelling; 酷 (cruel) + 薄 (thin/meager) suggests harshness.
Variant kanji form using 刻 (carve/cutting); less common but attested.
Compound of 酷 (cruel, severe) and 薄 (thin, meager), suggesting a lack of warmth or compassion. The variant 刻薄 uses 刻 (carve, cutting) with a similar figurative sense. The exact historical derivation is uncertain.