Who was responsible for Sirius' death?

[KathyK]

No: HPFGUIDX 167405

Bellatrix, Snape, Harry, Sirius, Dumbledore could all share some of
the blame for his death.

But I think we all know who is really at fault.  Clearly it's Grawp. 
It's great that he just happened to show up in the nick of time to
save Harry and Hermione from the Centaurs but did he have to go and
bleed all over the place?  If he'd just kept his blood to himself
those thestrals wouldn't have conveniently appeared to fly the Harry
and co. off to the Ministry.  Without thestrals, they may not have
made it to the Ministry at all.  And then Sirius would have no reason
to rush off and try to protect Harry.  

No blood, no thestrals, no Ministry, no duel, no veil.

Yep, all Grawp's fault.

KathyK, also considering Hagrid may have had something against Sirius
and that's why he brought young Grawpy home