sound of laughter, bad, malevolent
嘎 centers on a harsh, unpleasant vocal expression: the sound of laughter that is mocking or cruel, and by extension a malevolent or bad quality.
嘎 combines 口 (mouth) with 戛, which likely contributes the sound and may relate to a harsh or striking noise. The exact historical development is uncertain.
A mouth 口 making the sound of the right side 戛, like a sharp, grating laugh. That sound is bad and malevolent.
For カツ, imagine a villain letting out a harsh laugh that sounds like a sharp 'katsu!' as they mock someone. The cutting sound 'katsu' cues カツ.
—