Well, I can make up an answer: perhaps he needed a wizard still standing in the area because, um, because when the authorities arrived they would be able to tell somehow that the person who did this was still living.

Of course, that's idiotic, because really if the authorities had, you know, a grain of sense, they might have done something crazy like have Sirius take veritaserum or put his memory in a pensieve of be subjected to legilimency or or or or...

Heh. But it makes as much sense as any other explanation I've thought of so far.

Maybe Peter thought he wouldn't do a good job of standing up to the scrutiny?