There is something that never seemed to fit in here. lets see...
1 - when you enter the world, there is no hallow in it. only the regular natural terrain, and the corrupted areas. i suppose corruption is supposed to be DA EVIL THING OF DOOM. obviously it is extremely hostile to the player (heck, IMO its the hardest non-hardmode biome excluding the dungeon and Hell) and i believe even in Easymode spreads slightly, at a sluggishly slow pace.
2 - when you slay the Wall of Flesh, it says, "The ancient spirits of light and dark have been released", and thus appears the Hallow.
but exactly what IS the Hallow? the corruption seems to simbolize pure evil, the corruption of all things, decay, and death; while the hallow simply pressuposes happiness, and rainbows, and unicorns, and rainbows, and pink stuff, and rainbows. this should probably make the Hallow to be the exact opposite of pure evil, the PURE GOOD OF HAPPINESS AND RAINBOWS (i do realize i sound completely gay at the moment. you are allowed to laugh. i wount judge you), except for one thing: WHY ON EARTH DOES THE HALLOW WANT YOU DEAD SO BADLY?
it makes no sense, seriously. unless the Terrarian we play as is a complete sinner, he shouldnt be met with such hostility. yet, the same hostility is shown in the corruption, as well as, for that matter, in any biome.
3 - CONCLUSION: the world does not like us! and that is WHY mobs never attack eachother, they hate us players too much.
now on a more serious note, i realized that corruption mobs kill critters. maybe unaware, but they at least are able to damage them. when it rains during a blood moon, oh, so many rabbits are slain by the zombies. it COULD be interesting, and perhaps not too confusing to program, if there were a lot more different harmles criters based on other biomes, that were hostile against eachother or not.