Is the questioner sinful if the cat that was released is harmed because it is unaccustomed to life on the street, given that she was the one who caused its release?

There is no harm in releasing a harmful cat, as sheltering it is not obligatory, and one who releases it does not sin if harm befalls it. Sin falls upon whoever tortures it or locks it up and withholds food and drink from it, as in the hadith of the woman who was tormented because of a cat she imprisoned until it died.