@misadventure
See, I don't see emotionally supported heroes when I re-read that book.
What I see is a bunch of just-barely-not-kids called adults by the standards of the day. The first of the men is imprisoned, tortured, and presumably sexually assaulted, possibly by multiple individuals. Coming home, he brings his trauma and newfound xenophobia with him to this friend group, where the charming and charismatic central figure gets tangled up with the exact kind of person that they feel is dangerous, and then gets a disease and dies, further cementing their trauma and grief as this group of relative strangers bonds about the one thing they have in common in the only way they know how, lead down a path of increasing conspiracy by a wacky doctor that not even the doctor in their own group takes very seriously.
But he gives them a plausible explanation, and presumably a means of catharsis. Except, as we know -- that catharsis never comes. They act on those fears and paranoia, and they feel just as tainted and weak and afraid afterward as they always had. Mina, in particular, is torn between two worlds -- the exotic and seductive world of the strange and foreign, or the demure role of the housewife to the successful man that she genuinely cares for. Mina's story is probably one of the most tragic of them all, as, in her own words, she never really finds what she was looking for, and always feels outside of herself.
And then the group goes back, finding every scrap of evidence they can find to support their story and justify these objectively terrible things they have done that cost some of them their lives in a quest for revenge and redemption and just general ablution, and weave together this narrative with questionable evidence and few other witnesses.
It's a dark tale, yes, but it's not dark because of the monster.