fear
惶 centers on a state of fear or dread, often with a sense of agitation or alarm.
惶 combines 忄 (heart) with 皇, which likely contributes the sound and may suggest a sense of being overwhelmed or alarmed.
The heart radical 忄 beside 皇 (emperor) suggests a heart trembling before the emperor: fear and alarm.
For コウ, imagine a fearful courtier kowtowing before the emperor: kowtow -> コウ, and the heart trembles.