残酷
ざんこく · zankoku · JLPT N1 · na-adj
cruelty
残 (ruthless) + 酷 (harsh, severe) = doubled harshness: cruelty
残 means ruthless or to wound, and 酷 means severe or harsh. Stacked together they stress extreme harshness, the core of cruelty.
残 — 歹 (bones/death) + 戔 (small/cut, phonetic)
A semantic-phonetic compound: 歹 (bare bones, death) gives the meaning of harm or ruin, while 戔 (two 戈 spears, suggesting cutting) supplies the sound. Originally meant to injure or destroy, later extended to 'remnant, leftover' — what remains after damage.
酷 — wine radical 酉 + announce 告 (phonetic)
A semantic-phonetic compound: 酉 (wine jar) gives the meaning side, while 告 supplies the sound. It originally meant 'strong wine' or intensely flavored liquor. The sense extended to 'extreme' or 'cruel,' and in modern Mandarin it also borrows the English word 'cool.'