The question you need to ask is: Is there a reason for the hate?
The thing to think about though is that we have no control over the emotions of others and so feelings are feelings. Sometimes they cannot be explained simply – other times they can.
People can feel hatred for someone because they did or said something that prompted them to feel that way. Maybe they were betrayed, spoken to horribly, mistreated, abused, cheated on – the list is endless for possibilities. And if they feel it is warranted to feel hatred, then that is their feeling.
If the hatred is out of no where, I would suspect some kind of past life connection. The person probably did something in a past life to warrant the hatred feeling but they are likely unsure of why they feel that way now.