Looking at some members of the canon of Saints, it looks like there is no provision that prevents someone from being Saint AND Scourge. So there’s that.
ISTM part of the heightened tension over Mother Theresa has to do in part with secular media and popular culture portrayals in which she gets held up as some sort of extraordinarily exemplary humanitarian beyond the strictly Catholic Church sphere – exemplified AND boosted by the Nobel accolade. Absent that flank by which she was media-canonized in life among many Western elites, she might not have become a candidate for Fast Track sainthood but her regular-order cause for canonization (founders of Orders commonly head that way) would probably be proceeding at a comfortable pace with normal pros and cons being argued. That also intensifies the spotlight on the issue of the existence and working of a “missionary order” into the late 20th Century when a lot of our mainstream culture has come to see missionary/proselytizing work as distasteful or even oughtright a wrongful act of aggression.
Many in the West even outside Catholicism too eagerly drank up the Mother Theresa image and mythos in life, and to many now it seems shocking or gratuituously hostile or negative to cut it down and point to shortcomings and failings. Which it is not. Her pop-media elevation to “celebrity living saint” may well deserve getting popped publicly and that would just be a matter of truth. But in the end, the cause for actual canonization stands or falls on the merits that are required for it: she’ll be canonized because she meets the RCC’s standards, not because she meets the standards of our movement of Concerned Utilitarian Humanist Rationalists (oh, and our CUHR’s opinion plus 5 Euros gets us coffee at the Vatican…)