I'm not generally an angry person (although sometimes I get angry at certain events or people), but I think it's completely reasonable for anarchists to be angry.
Why would an anarchist be angry?
Might it be due to injustice? Theft? Might it be that these vile things are accepted as normal, or even desirable, by the vast majority of people?
Might it be because he is a good person who sees immense evil being called "good" or "necessary"?
Why would anger under these conditions be unexpected?
Anger doesn't justify taking it out on innocent people, though. And that recognition is what makes us better people than the rabble who do.