We are sinners living among other sinners, and so it is inevitable that we will have to make choices that are related to the sinful choices of other persons. Our knowingly chosen acts often have some relationship to the sinful acts of other persons. Moral theology uses the term ‘cooperation with evil’ when considering whether or not we sin, if our knowingly chosen acts are related to the sins of other persons. Cooperation with evil refers to any situation in which one person is sinning, and a second person must decide whether or not to commit a related act.
Despite the term ‘cooperation with evil,’ our acts are not always sinful merely by being related to the sins of other persons. If that were so, then nearly all acts would be sins, for we live as members of one human family, in one world, and all our acts are in some way at least distantly related to the acts, even the sinful acts, of other persons. So it is true that sometimes we act without sin, even though our act is related to the sin of another person.