since @TakeN is so inspiring, here's another #riddle for your sunday evening:

As I do my job
- I kill people sometimes
- But I serve and help people more

As I don't do my job
- I first seem to help people
- But in the end, my help is sure death

I'm either doing my job or I don't.
I cannot go back and forth as I please...

What am I?

(hint: the answer is NOT "vaccine") :-)