è is the workhorse reading - every 'evil / vicious / fierce' compound: 恶意 (malice), 恶劣 (vile), 恶毒 (malicious), 恶魔 (demon), 恶性 (malignant), 邪恶 (evil), 罪恶 (sin). For the 'to hate' verb sense, switch to wù; for the 'nauseous' sense in 恶心, switch to ě.
memory
Top 亚 (second-rank, lesser) sits above the heart radical 心 - a 'lesser heart', a corrupted mind. The phonetic gives the sound (yà → è), and the semantic bonus reinforces the meaning: evil is the heart fallen short of good.
register
The 'to harm' sense is archaic and literary; it does not appear in modern Chinese, where è is used only for the 'evil / vicious / fierce' meanings.
Bottom heart radical - the indexing component. Marks 恶 as a mental or moral quality: hatred, evil, vice. Same heart-base anchors 怒 (anger), 思 (think), 忘 (forget), 急 (urgent). The reading è = evil; reading wù = to hate; reading ě = nauseating.
Top 亚 supplies the sound - yà drifted to è with initial loss. 亚 itself depicts a tomb shaft or building foundation, traditionally read as 'second-rank, lesser'. The faint semantic echo of 'lesser' reinforces the 'bad, evil' reading è. Same phonetic in 哑 (mute), 鸦 (crow).
