The scene went like this:
"You're going to kill me?" Harry choked, attempting to pries off the metal finger. "After I saved your life? You owe me, Wormtail!"
The silver fingers slackened. Harry had not expected it: He wrenched himself free, astonished, keeping his hand over Wormtail's mouth. He saw the rat like man's small watery eyes widen with fear and surprise: he seemed just as shocked as Harry at what his hand had done, at the tiny, merciful impulse it had betrayed, and he continued to struggle more powerfully, as though to undo that moment of weakness."
IMO it was a little of both. He committed indirect suicide by showing that one moment of remorse and it was murder as he didn't want this to happen.
I cant say I agree completely here...see Suicide is killing ur self(against or with ur will it doesnt matter..But accepting the death as inevitable)--Petigrew didnt Kill Himself...that One moment of mercy and gratitde had indeed Betrayed him---and how? he had narly disobeyed the man who gave him the hand and with that own hand!--N so He had o pay the price...the hand killed the man who wud have failed the master's bidding...