First of all the MC is NOT a boy. He is a grown post collegiate, over the age of 21, who is holding down a full time professional job living apart from his parents.
Second, while is probably true that most of us have people in our past and or present whom we'd claim to hate, how many of them raped an underage member of our immediate family, got away with it, and based on our observations has done the same to others over the years? That may create a response rather more intense than hatred. Out of the broad distribution of people and their possible responses, some percentage would elect for direct intervention to correct Klaus waking away. We can debate whether or not that falls into an "adult" response, but that's not relevant to the choice to act or not.
The events in Victor's life certainly could have sensitized him to react as he did when it happened to him and his family before, generating a reaction that considered more calmly wouldn't have occurred. We're given only the slightest insight into that event in Victor's life, so we have no real basis to judge if his offer to the MC was believable, except he availed himself of the option at the time.
you are right, I have oversimplified
but being an adult is not an anagraphic fact, MC is for the first time outside the family home, for the first time he is really the master of his actions. moreover, he is facing an extraordinary situation.
Victor, on the other hand, is more experienced, and has had the misfortune to find himself in a very similar situation. in fact, at the beginning, the advice he gives to MC is very mature, he prepares him for the worst and makes it clear that it cannot be him who helps Haley, but that he needs a professional, a therapist. up to that point, he is very mature.
in that moment mc knows very little about what happened to his sister, practically nothing, that's why showing him the shortcut right away is really very risky on Victor's part. Victor goes from being a positive mentor to a tempting devil.
but also narratively the whole parenthesis of the gangsters is a foreign body to the story, there is no continuity between the visits to the second Haley, Haley's self-awareness and the brutal shortcut that MC decides to take.
