It depends on which side you are on, or where you prefer to place the novel: science fiction-esque or supernatural fantasy.
JAL combined the two, which makes the novel so good.
In more "sci-fi" territory, Eli is just a boy/girl with an unusual illness, and is supposed to be a little LESS powerful, the character should more or less work with the actual natural laws of this world, which would explain why he needs adult help, maybe even if he is stronger than a "normal" person, he will still have problems with stronger adults. Here everything is more literal and social, materialistic let's say.
In the supernatural realm, Eli is more of a cursed person and his power can be unlimited, the character has his own rules (the rules of the genre, or vampiric rules) in this case Eli is more ethereal and romantic (that is, romanticism or expressionist, a tortured person in the heart ). A supernatural being in a natural world. So everything is more symbolic, including Hakan's dilemma and his zombie return.
The Eastern European "original" vampire, if I'm not wrong, is essentially a very zombie-like "undead" or living dead, a person who died a strange or non-Christian death and therefore comes back to life, sucking blood at night. Like a soul that could not "rest in peace."
Eli never died, she should have died because after so many years. So you could say that she is half human, half "vampire". UNLESS we assume that Eli died after the lord tortured her and then came back to life as a vampire, in which case Virginia should have died before becoming a vampire as well.
I think originally, a vampire sucks your blood night to night and obviously you get sick and then you die, there you become a vampire. Not before.
If vampirism is a biological infection, zombie-Hakan is basically a corpse with a parasite in command, another explanation for why he is a mindless monster, is because he literally has no brain, after the fall his head was screwed. It's like he's in a coma or has severe brain damage. But that doesn't explain why he is so savage in contrast to Eli. Zom-kan is a very grotesque flesh-eating machine, we don't know if he is rapey to any young boy/girl or he is JUST after Eli tho.
I suppose that if Hakan instead of jumping from the fifth floor had broken his neck (like Jocke), he would be reborn more as a "human" vampire (like Eli) BUT Eli also says in the novel that a "revenant" is more dangerous, which I suppose means that those who actually die and come back to life as vampires are almost monsters with no trace of human soul.