忄 is the vertical-heart radical, the left-side form of 心. It frames 怪 as an inner reaction: finding something strange is a feeling of surprise or unease, and blaming someone is a mental judgment. Same radical in 怕 fear, 情 emotion, 忙 busy, 怀 cherish.
Right side is 圣 — visually 又 (hand) over 土 (earth), historically a separate phonetic graph distinct from the modern 'sage' character that shares the shape. Supplies the sound: an Old Chinese reading near *kruːls produced guài, with the phonetic match looser in modern Mandarin than it was in antiquity.
