Why it had to be Sirius

[that space cadet glow]

No: HPFGUIDX 75664

My theory on why Sirius had to die has to do with the fact that for 
the first time, we really see Harry screw up and fail. It's because 
of Harry's actions (as well as DD's) that Sirius wound up being in 
the DoM and getting killed at all. We saw Harry fall into a trap and 
that's a learning experience for us as well as Harry. We learn that 
Harry isn't always a perfect hero and Harry learns that being rash 
can have deadly consequences (I hope that's what he winds up 
learning, anyway). 

~dream