Well, of course he did. Lots of people will have been telling the police that there was public bad blood between Matt and Justin. To be surprised by this argues that Justin could really use some thinking-things-through lessons; he should have been expecting this and had an answer ready! To be “in a foul mood” as the result of learning it argues that he could also use some Coping Skills.
There is a reason that he has been re-named JustAn Idiot, you know.
Being in a foul mood and wanting Revenge/to Crush the nasty person is his default setting unless everything is going exactly as suits him. (I think this may be a symptom of some sort of psychopathy but cba to check.)
Gah. It reminds me of that bit in cop shows where someone says “am I a suspect”, and the cops for some reason never reply “yes sir, it’s entirely routine, everyone is a suspect at this stage of the investigation”.