The dog you roll on the bed with that licks your face, will under different circumstance rip your face off or your throat out without much thought between the two.
I think that JAL suggests that Eli always existed between brute animalistic drive and quasi-civility and that there wasn't much needed to switch her between the two modes regardless of who she was with, or how much or how little she cared for them. There are numerous instances in the novel where Oskar was looking at a good numming from Eli's animal instincts, even though she loved him dearly.
One could say that the only way she could ever protect Oskar from what had befallen Hakan, who she also loved, was to turn him into one of her own kind for good.... through marriage. And it doesn't matter if this was an ultimate act of selfishness or the ultimate act of love because the two are so intrinsically entwined.
The fact is that there was no future for either on them together while Eli was the hunting animal subjected to brute instincts and Oskar was human. I would say that both of them made the perfect choice... seen only out of mutual love.