On-screen decapitation would probably be considered mature, yes.
The problem with the "can you show it to your mom/boss?" thing is some people have really prude moms or bosses. Hell some of them think the gay agenda is a thing, or that a short skirt deserves a mature rating to be kept away from the innocent little children so they dont -gasp- masturbate!
It's probably easier to think in stricter terms of what is actually going on on-screen.
I think fist-fights, slaps, and the like would make it without a mature rating just fine.
Chopped off limbs, body parts, graphically detailed gunshot wounds or excessive amounts of blood (say a scene with a dead body that, while it doesn't have any graphically detailed wounds, is lying in a pool of blood with blood smeared all over the room indicating a struggle) might need a mature rating to avoid making some readers squirm.