why did Nina run the car off the road?

Carlos told Nina the truth about him, but she didn't believe it. The only way he could convince her is to lie and say he liked all the others more than her (including her sister), to tap into her insecurities. She was most likely trying to kill them both, or more so preventing Carlos from "craving" anyone else but her.

I think he told her the truth because he hated himself and didn't think he deserved to be happy (or knowing he couldn't curb his cannibalism). He actually did care for Nina, but wanted to drive her away. Ultimately leading to the ending, which I am sure neither of them really wanted.