Um… so many reasons… and I haven’t read that book in awhile so I’m not the best resource for this. The fandom tends to hate on Benji bc:

• He’s a shameless Gary Stu
• He’s a child, maybe 12? Marius knows better (without spoiling anything, I’ll leave it at that).
• He smokes (not a character FLAW per se, but it seemed to be a cheap way to make him look “tough kid” or whatever)
• Emotionally/intellectually he’s flawless! He’s such an angelic person! He can do no wrong! Such characters are inherently unlikable maybe bc we can’t suspend disbelief for someone so PURE OF HEART. 
• The way he talks seems oddly teenage? I remember someone saying that as a complaint.

Mostly we hate him because (and this can all be applied to Sybelle equally) of his existence, how he came into Armand’s life, and what happened to him seemed wildly out of the VC universe and out of character for everyone involved.

Here have some fanart:

Co-existing by luizagm2