Why Did Sirius Play the Joke on Snape?

[cynthiaanncoe at home.com]

No: HPFGUIDX 24792

In PoA, we are told that Sirius plays a "joke" on Snape that could 
have gotten Snape killed by following Lupin to the Shrieking Shack.

I have always had trouble with this.  Lupin was lucky to be admitted 
to Hogwarts in the first place, and elaborate protections have been 
put in place so Lupin can attend school.  Wizards don't like 
werewolves, and Lupin is keeping it quiet that he's a werewolf.  
Lupin is one of Sirius' best friends.  Sirius knows all of these 
things, is smart, and is at least a fifth year student. 

So why in the world would Sirius deliberately set up a situation 
where Lupin, one of his best friends, kills someone and would thereby 
be expelled or worse?  And why doesn't Lupin show any signs of ever 
having been at all peeved about being used like this?  

Any thoughts?

Cindy