Black vs. Pettigrew: Why was Black Laughing?

[cynthiaanncoe at home.com]

No: HPFGUIDX 24603

When Black hunts down Pettigrew in the street, Pettigrew blasts 
himself and a street full of muggles.  In PA, we are twice told that 
Black stood there laughing when this happened, and was still laughing 
when he was arrested.  (Stan on Knight Bus and conversation in 
Hogsmeade).

Why would Black be laughing?  Thirteen innocent bystanders are dead, 
he has learned that his old friend was a Dark Wizard spy who betrayed 
Lily and James, and he recently learned James and Lilly are dead.  He 
must know at this point that he'll be blamed for the betrayal (he 
won't be needing the motorcycle anymore).  What's so funny about 
that?  

I haven't heard this discussed before, and I can't figure it out.  
Any help out there?