Exactly, this is why it's so confusing. There are only two real possibilities, and neither one makes sense.
Iron Islanders: wouldn't get the amnesty if they slaughtered everyone and burned the place down. Doing so would seem to lead to certain death.
Whatsname's bastard: serves the Starks, and even under siege conditions presumably wouldn't burn down the place he's trying to retake.
Other possibilities:
-Winterfell residents (why?)
-third party we haven't met yet (diabolus ex machina, boo)
-Hodor Hodor Hodor
Iron Islanders: wouldn't get the amnesty if they slaughtered everyone and burned the place down. Doing so would seem to lead to certain death.
Whatsname's bastard: serves the Starks, and even under siege conditions presumably wouldn't burn down the place he's trying to retake.
Other possibilities:
-Winterfell residents (why?)
-third party we haven't met yet (diabolus ex machina, boo)
-Hodor Hodor Hodor