what do shinigami do?
shinigami "exorcise/purify" (祓う) marks (的[テキ] (TEKI; mark, target) / 敵 (TEKI; enemy) which are either believed to be "enemies" like it is your life's purpose to despise or harm these marks or "aims" like it is just your mission objective from the higher-ups in shinigami society, which is "the society of lesser gods" (従神社会; JUGAMI SHAKAI)
(colloquially by shinigami referred to as "the society of curse gods" (呪神社会; JUGAMI SHAKAI))
to exorcise / eliminate these marks.
the "marks" are "ヒトビト" (HITOBITO; people) but not "人間" (NINGEN; humans) which are different. hitobito can be converted to shinigami / transformed into such upon the moment of purification / exorcism if they try to kill the shinigami trying to exorcise them, thus awakening their spirit pearl (霊真珠; REISHINJU).
but 人間 (NINGEN; humans) or 一般人 (IPPANJIN; ordinary people) will never be able to become shinigami.
you either die human or become a god