Humbert was an actual paedophile who had gone out of his way to find young-looking prostitutes to sleep with for the entirety of his life.
He initiated a relationship with the mother of Dolores for the sole purpose of keeping himself close to the girl, and when the mother died he went out of his way to ensure he could get custody, despite not being the father.
I don't remember every detail now, but I'm fairly sure he had to go on an improvised road trip after handing incomplete documents so that authorities would let things go, and as soon as this trip started he had sex with her 12-year-old self in a run-down motel. After which he kept abusing her well into her teens, to the degree that, despite having had fond thoughts of him in the past and him being the only one she could rely on, she chose to run away with another old man who only "saved her" so he could do the same.
And at the end of all this he found this man and killed him.
Oh, and did I mention that for the whole duration of this debacle he kept a diary in which he fashioned himself a decadent poet and wrote sweetly of his twisted love?
Comparing Kousuke to Humbert is like comparing Trump to Hitler.