Why Do People Lash Out in Anger at Others?

I just sent out my March Newsletter 

and this morning and just got this angry message to my website:

He left a fake email and most likely fake name

He calls himself Justin, so this is my public reply to him:

Dear Angry Man,
I send out newsletters to my subscribers, because they SUBSCRIBED and are interested.
I don't collect emails through tricky giveaways or through unclear, misleading methods.

The only two ways anyone can subscribe to my newsletters:
1. If they visit my website and click on the clearly written pop-up window to subscribe.
2. They read my newsletter by clicking on the posted link on social sites and and click the subscribe button.

If you subscribed by mistake or no longer interested in receiving my newsletters, please scroll to the bottom of the newsletter and simply click the UNSUBSCRIBE button. 
Using foul language and marking my newsletter as spam is uncalled for when there is a way to behave in a civilized manner.
Thank you!

What could be the reason of his anger?

Is it because anger feels better than feeling sad, miserable, or desperate?
Anger gives us a temporary feeling of false power.

Is it because hiding behind a fake name gives him the freedom to say whatever he wants and hurt anyone who happens to be in his way without consequences?

Is it because he's powerless to control his own life or maybe he feels entitled and angry at the world because the things he wants are not handed to him on a silver platter?

I wonder...
What do you think?