I'm writing a story where the antagonist is motivated by lust, and has successfully engaged in predatory behavior, the antagonist is attempting to become a powerful mage in order to solidify his paradise and use magic to make his experiences more vivid. The protagonists have trusted the antagonist the entire time and their friend even committed suicide as a result of his actions.
I'd like to solidify his emotional impact on the characters in a natural way and therefor do not want him to jump into an unprovoked monologue, but I can't seem to think of a natural way of doing this.
I have considered both the scenario where the main characters learn everything at once when they discover who the antagonist is, and I have also considered (and think it would be better) for the protagonists to slowly realize all the things the antagonist has done before they actually find out who he is. unfortunately I can't think of a reasonable mechanism by which this should happen owing to the inexperience of the main characters.
Rather than a monologue or something that feels contrived, what things can I consider to introduce my antagonist's misdeeds to my main characters?