die
歿 centers on the end of life: dying or passing away. It is a direct, somewhat formal kanji for death.
歿 combines 歹 (death) with 殳, which likely contributes a phonetic element. The exact historical development of the right side is uncertain.
The left side 歹 is the death radical, and the right side looks like a person collapsing. Together they show the moment of dying.
For ボツ, imagine a boat (ボート) sinking and everyone on board dying: boat -> ボート, and the short ボツ sound is like the boat going under.