Why are Slimes being massacred?

Slimes are being massacred for their resources, Gel: a fuel that is used for Torches,it is also used as ammunition for a flamethrower. Killing Slimes is just like excavating the earth for oil. Why isn't there an alternative source for fuel in Terraria? What about Coal? Or Oil? Why must we use the remains of dead beings to fuel us? (Dinosaurs don't count) Slimes are also being massacred because they attack some Terrarians, but that is out of self-defense, in fear of death by Terra Blade/Sunfury or anyother Endgame weapon.Even mechanical contraptions have been built for the farming of Slime Gel!The killing of a slime:Ah, that is when the Slime souls rise into Heaven, where it eventually becomes too crowded for souls there, requiring The Terraria God to ressurrect them and throw them back to Terraria, only to be killed off againA Slime Rain taking place: