If you regard everything people do that's maladapted as a mental disorder there would be virtually no one without one.Is veganism a mental disorder?
Human nature is the result of evolutionary pressure to survive, this frequently involves contradictions. For example we have an instinct to run away when danger threatens but also an instinct to stand and fight. these instincts express themselves in individuals randomly but are balanced on average in the population. Some people have too much on the 'run away' side, some too much 'stand and fight' just as some people are very tall and some are very short.
Empathy is a similar case, too little and society and even the family won't hold together, too much and you are over-tolerant of destructive or parasitic behavior in others and society is weakened.
In the particular case of veganism it's not so much about too much empathy as about empathetic instincts being misdirected. This is most likely due to experience. The vegans I've known have frequently been turned off applying their empathy to humans and misdirect it onto animals. Often this seems to involve ideas about 'guilt' and 'innocence' (the manifestation of another instinct), humans are generally less deserving of empathy because they are guilty where animals are innocent.
The amusing contradiction is part of their creed is a moral equivalence for humans and animals which would make such a distinction nonsensical.