behead, dismiss
馘 centers on the violent act of cutting off a head, literally beheading. The dismiss sense is a figurative extension: removing someone from a position as if cutting them off.
馘 combines 首 (head) and 或, which likely contributes the sound. The character originally depicted cutting off an enemy's head as a trophy in battle.
The left side is a head 首, and the right side 或 looks like a weapon. Picture a head being cut off with a blade: beheading.
For カク, imagine a sharp 'crack' sound as the head is severed: crack -> カク, and the head falls.