Very often people have their perceptions distorted by their emotions. When someone calls someone a "hater" they are saying that the "hater" is a person who lets their perceptions get distorted by hate, or some other source of negativity that is being channeled into hatefulness.
Usually that accusation is justified. Even if the complaint itself is reasonable, in most cases, the act of complaining is not. For example, if you see someone who is overweight, the sentence "you are fat" is arguably objectively correct. However, the act of saying that sentence out loud reveals something about the speaker. The weight level of a stranger is not their business. They do not convey any new information. It seems likely that the motivation for making such a statement is some desire to hurt the stranger. Desiring to hurt strangers is a bad thing, and is pretty much the definition of "hater". A "hater" is someone who lets their own personal emotional problems colour their perspective of the world, generally seeing other people in a negative light, and generally thinking it is okay to hurt strangers for no reason. The world is chock full of people like that and if you can't see that, it's probably because your perspective is similarly skewed.
To clear up a few things, context is important. It shouldn't be socially acceptable to publicly throw vitriol at strangers who are deemed attention seekers. (If you don't like attention seekers, don't give them attention. Nothing more is reasonable.) On the other hand, someone singing badly outside your apartment clearly directly affects you. People who campaign against fox hunting are personally invested in the issue. Whether or not it's reasonable for them to get upset about it is a matter of opinion - and is closely related to whether or not their views are deemed reasonable. The anti-fox hunting movement is generally seen as legitimate, even by people who disagree with them. However, not everything that people campaign on is regarded as reasonable, take the Westboro Baptist Church for example.
Being turned into a hipster meme is pretty harsh, whatever you say. Even the hardiest of Internet tough guys break down under torrents of social media abuse. There are countless examples of Reddit mob justice singling people out and breaking them down. It's obviously a deeply unpleasant experience, and unless you've been through it, I don't think you're in a position to make light of it.
Lastly, I just want to say I find it hard to understand why you don't have any concept or understanding of what bullying is. The most likely explanation is that you are prone to negativity and you're being self indulgent about it. So, given your commitment to absorbing other people's perspectives, why don't you take on board mine? I think you're probably an overly negative person. If you see the majority of people around you negatively, then you have unrealistic standards (by definition). Unrealistic standards will make you unhappy, because you will be perpetually disappointed and full of complaints. It's better to care less about the faults of others and enjoy their good points. In fact, if you want to live a happy life, then you need to have tougher standards on yourself than you do on others. That might sound counter intuitive, but it's pretty obvious that most people live pretty unhappy lives. If you want to live a happy life then you need to do better than them - and you can't get upset at their failings, because that'll make you unhappy.
