While it is easier to be mean online, it is also easier to be nice. On the internet I like to be very lighthearted and optimistic, and I find that many people IRL do not appreciate this. Also, you can be nice to other people by helping them do something, building something, whatever, and then you can dispose of an identity easily if you get emotionally attached to it. IRL if I did one thing or another, it would make me anxious to feel obligated to keep up a reputation whose surface area just keeps growing excessively (in terms of the number of words/actions), people may rely on you or expect you to act in a particular way. With anonymity you can mitigate this.