Left person radical - the indexing component. Debt is fundamentally a relationship between people: one owes another. 亻 marks 债 alongside other interpersonal verbs and nouns: 借 (borrow/lend), 偿 (repay), 信 (trust), 仇 (grudge).
Right 责 supplies the sound (zé → zhài, retroflex shift) and a tight semantic fit - to hold responsible. A debt is exactly that: a person + a responsibility owed. Phonetic family includes 绩 (achievement), 啧 (click of tongue) with related readings.
