Pillage was the usual practice when a garrison didn't give up, was it not?
Further, the prisoners executed after the capture of Jaffa were, in large part, paroled prisoners who had already surrendered once and released. If they took up arms again, according to the 'rules of war' of the period, they were subject to execution if recaptured.
That set of facts is usually overlooked, especially in Dwyer's article if I recall correctly.