The ě reading is bound to one compound: 恶心 (nauseous; disgusting; to deliberately gross someone out). Note that the very same compound 恶心 read with è means 'bad habit / vice' - a rare and literary sense. Modern speech almost always intends ě / nausea.
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).
