My feeling is that it's either subjective or dependent on context, but I can't decide which. Maybe it's both.
In the context of a religious debate, the whole "hating God" thing is a red herring to try and direct the conversation away from what is demonstrably real, or whatever point you're driving at. So you'd say that you can't hate something you don't think is real because you want to avoid going off on that tangent.
In fiction, I suppose it's best to say that you hate the character, real or not. You might hate the character "God" or the character "Jar Jar," but in the end it is just the character. And then I think it would have to come down to where you choose to direct that hatred. Clearly Jar Jar is George Lucas' fault, but if you don't think about it in those terms I don't see why it would be any more or less accurate to say that you hate Jar Jar, because that's how you perceive it.
This is where words begin to fail me, so I hope this makes sense. If I see the character as an idea, and an idea is a real thing even though the thing it represents isn't, then I can say that I hate the character and refer to it by name. That doesn't mean that I believe the character actually exists, which I suppose makes it somewhat hypothetical hatred.
But to the believer, God is a real person, not a character, so they aren't talking about hatred in the same way that you are. So it would make sense to say that you don't hate God because you don't believe in him, because the context is different. And I think that the ambiguity of where hatred is directed when you talk to a believer would give a false impression of what you actually think unless you take the time to explain it, which is exactly what you don't want to do when it's irrelevant to the conversation.