Earlier, Mr. Collins was characterized as "not vicious" when he and Charlotte became engaged. I suppose at the time this did not have the meaning it does not, i.e. dangerous and violent or just generally bad-tempered; it was probably something more like not having vices such as drinking to excess, gambling, womanizing, etc.

But he does have a thoroughgoing nasty streak that would make me fear for Charlotte if he weren't stupid enough to be fairly easy to control by a woman of Charlotte's tact and shrewdness. Think of it: he made his proposal to Lizzy and was almost literally deaf to her refusal, not to mention pointing out (in so many words) that she'd better take him because she might never get another offer; when Lizzy visits Hunsford he actually expects her to show signs of regret at what she's lost by refusing him, and now that letter about Lydia, with his comment about how it's just as well Lizzy refused him: "And this consideration leads me moreover to reflect, with augmented satisfaction, on a certain event of last November; for had it been otherwise, I must have been involved in all your sorrow and disgrace."