Since God's creation is good/perfect, how could a creature as good/perfect as a top angel actually fall, according to Catholicism? What is the mechanism of corruption that can corrupt a perfect being? Since God is incorruptible, He is then still the absolute perfect, while any other creature with free will cannot be absolutely perfect. But then, what is the guarantee that humans or saint angels will not fall again, being already in heaven?
Angels, incorporeal intellectual beings, cannot become less perfectly angelic (unlike humans, who can become more or less human); also, every angel is its own species. However, there can still be "evil of fault" (malum culpæ) in angels, not in their intellects but in their wills. Angels have freewill.