If you wanted to take it a step further, you could argue that water benders could take out all the fluid from someones body, turning them into a mummified husk 

and a highly skilled metal bender could control the iron in another person’s blood, kinda like what Magneto does in X-Men a lot

What a great time to be anaemic.

earthbender ambushes

waterbenders surfing through the battlefield on a wave of blood.

"on a wave of blood" 

but yes, let us continue with this gore fest!

Is there possibly a way that firebenders or airbenders could raise the body temperature of an individual to the point where your burning/melting from the inside out?

or what about earthbenders being able to break bones since bones are made up of various metals of course along with non-metals but that’s beside the point?

god bless this fandom. we’re all growing up and turning into psychos

but earthbender’s entombing people in rock sarcophaguses or torture of being slowly crushed by rocks 

Yeah. ANY form of bending, if used properly could lead to a horrible, gory death

Metal benders could use just a tiny ball of metal, the size of a pea, and just zip that thing straight through someones’ skull