It still sounds like his moral compass points north just fine.
Wierds? Hit and miss. If the circumstances were right, he would feel perfectly OK cancer-killing somebody... or even multiple somebodies.
Think "mr. Spock", with emotions, and killer magic.
The logical, rational component of his psyche is doing double duty standing in for a missing concience. He "knows" certain things are "wrong", but is more academic now rather than intuitive. He feels thari's books should be either locked up in a bullet proof case and strictly controlled who gets access to it, when, where, and why-- or that they should be destroyed immediately. He has determined this on a purely logical foundation, that the majority of their contents deal with subjugating and injuring other people, and that improperly controlled, that knowledge will lead to social catychisms of epic proportions.
The idea of causing the suffering does not illicit a strong repulsive emotional reaction. Instead, it illicits a kind of emotional "memory". He remembers being disgusted by those sorts of things, and is now merely puzzled by his memories. To understand his puzzlement, he rationalizes why he has such a recollection in the first place, and does a "cause/effect" breakdown.
Using the cancer-curse on somebody wouldn't make him flinch emotionally. Instead, he considers if doing so is really in his best interest. Using it on the dragons outside would certainly be effective. However, the fortress residents would see it, and would treat him differently. He knows academically that it would be a war crime, and is inconsionable. He knows he should be disgusted by it. To make up for the lack of intrinsic disgust, and to hold on to his humanity, he immerses himself in those remembered reactions, as a proxy. His memory is really good, and he willfully remembers such feelings of disgust as vividly as possible. Its the best he can do.
This lack of intuitive repression against abhorrent thoughts and acts is what drives necromancers over the edge. They quite literally see nothing wrong with killing people.
Weird is a very young necromancer, and hasn't had time to become apathetic and jaded about his emotional handicap. Add to that the fact that he is aware of the trap, and conciously works to overcome the problem.
If hugo is willing to be glued to a reluctant sociopath, who remembers not being one, and who doesn't want to be one, the connection would be mutually beneficial. Weird would gain a more balanced personality, and hugo might suddenly find himself with new innate powers from the connection.
It would also make corai angry about not being able to kill him. That alone would be worth it.