I have a theory.
I think it's because they act pathetic and openly display their weakness, which triggers a primitive urge to shun and hate them.
In Mankind's early history, and in a lot of animal packs, weak members of the group are shunned, euthanasised or left to die for the good of the group.
When we see people openly displaying their weakness it triggers a innate hatred for them because we subconsciously think they're a drain on our group and detrimental to the survival of our species.
Emos are like the polar opposite of alpha males, but infuriatingly it's not because of any disease, injury or birth defect, it's by choice, which makes most people subconsciously rage and hate them... they could be successful and productive members of the tribe, but instead they choose to act weak and wallow in self pity, and to make matters worse they whine and complain about it, blaming the rest of the tribe for their self inflicted pathetic state.
It's like Emos are trying to oppose nature's rule of 'Survival of the Fittest' which, like many other things that we perceive to be unnatural, we abhor.