It is very clear that he can be much more dark and scheming than he seems to be. That is illustrated by just how far he will go to possess Lolita - marrying her mother and then literally abducting her after her mother dies. In addition, they both are tragic figures who never get what they really want. Humbert discovers he is capable of love, and that he loves Lolita, even when she is "passed her prime" at 17. He says to himself, "[a]nd I looked and looked at her, and knew as clearly as I know I am to die, that I loved her more than anything I had ever seen or imaged on earth, or hoped for anywhere else" (Nabokov 279). Humbert is capable of real love, or as real as it can be for him, at least, while Quilty seems to be an incarnation of the devil, an evil presence in both Humbert and Lolita's lives.

This evil is illustrated by his address, "Grimm Drive," and the nonchalant way he treats Humbert, even after he threatens to kill him. They become one at the end of the novel, indicating that without Lolita, neither of them are whole or complete, and so they have to meld together with their similarities to even survive. Nabokov writes, "He was naked and goatish under his robe, and I felt suffocated as he rolled over me. I rolled over him. We rolled over me. They rolled over him. We rolled over us" (Nabokov 301). The two characters become indistinguishable,...

Humbert continues, "And do not pity C.Q. One had to choose between him and H.H., and one wanted H.H. To exist as least a couple of months longer, so as to have him make you live in the minds of later generations" (Nabokov 311). Thus, while they seemed quite different at first, both these men were very much alike, and they merge into one sad, pathetic character at the end of the novel. They both possess Lolita for a time, but in the end, neither of them can have her, and they are shown for what they really are - sick, useless men who are aging gracelessly and without any social or moral purpose in life.
In conclusion, the two men serve as doubles in the novel to make it twice as clear how pathetic and disgraceful Humbert is. Quilty is the dark, evil side of Humbert, indicating just how awful he could be if he let this side take over. In the end, they are both destroyed because they are oddities - men who love little girls, and they should not exist in society. Nabokov makes them funny, pitiful, and strange at the same time, because those traits mirror their lust and obsession with young girls, and the two of them together make their crime even more disgusting and sick.

References

Nabokov, Vladimir. The Annotated Lolita. Ed. Alfred Appel, Jr. New York: McGraw Hill, 1970.