beheading
剄 centers on cutting off a head: beheading. It is a specific, violent act of execution by blade.
剄 combines 巠 (possibly phonetic) with 刂 (knife), suggesting cutting with a blade. The specific meaning 'beheading' likely arose from this knife-related action.
The left side 巠 looks like a neck with a head on top, and the right side 刂 is a knife. Picture the knife slicing through the neck: beheading.
For ケイ, imagine a beheading scene where the executioner says 'Kay!' as the blade falls: Kay -> ケイ.
committing suicide by slitting one's throat