No evil can be excused because it is done with a good intention.
Nullum malum bona intentione factum excusatur.
Original text (Latin)
Variants
An evil action cannot be justified by reference to a good intention.
Original text (Latin)
Charity, by which God and neighbor are loved, is the most perfect friendship.
It must be said that charity can, in no way, exist along with mortal sin.
Quote text and notes from Wikiquote: Thomas Aquinas (page history), available under CC BY-SA 4.0. Source quality and labels are our own summary of what Wikiquote records.