Spock's "logic" has a theoretical and practical component. The practical component appears to be a utilitarianism to some extent constrained by deontological rules, in particular the duty not to kill the innocent and the duty to be faithful to commitments expressly undertaken, such as to the Federation. The other characters criticize him for lack of "emotion". In the theoretical context, this largely refers to the inability to predict the behavior of others (and occasionally maybe of self) due to a lack of emotional imagination (I am sceptical whether emotional imagination is needed to predict the behavior of others, and I think a psychopath could be very effective at predicting others' behavior). In the practical context, this seems to refer to a failure (and not a total one, since he is part human) to be moved by certain kinds of reasons, in particular reasons of friendship that go beyond commitments expressly undertaken.