How/Why Did Eve Become Good?

Because the script said so. Considering what they did to her I would have said *beep* you guys*

It's never really explained. In my own head canon (because I spend way too much time thinking about this series of movies) and assuming you mean at the end of the movie, I believe that Eve never actually "went bad". After her encounter with Patrick I think that, in addition to being overwhelmed with mating instinct, she realizes that he is going to destroy the human race.

Earlier in the movie she tries to remind Laura that she is in fact human, if only partially. Plus she was willing to suffer through horrible experiments so that humans could be prepared if the aliens ever decided to invade. I believe that she actually really loved people and that's why she broke out. The psychic link showed her all the cocooning offspring and she realized that she had to act quickly.

So she breaks out but allows the team to track her. She could have easily ducked off into the woods and evaded them but she took a marked truck and stayed on the road so they could follow her right to Patrick's barn. Now she was still very much acting out of mating instinct but I like to think her human side affected her actions, particularly letting them follow.

Once she got to the barn the instinct was too much and she had to give into it. But that also earned Patrick's trust and allowed her to get in close to him. I'm not saying it was entirely a premeditated ruse but maybe she hoped to get in close to Patrick (distract the guy with sex, sure to work) and hope that the team got there quickly. Then either Laura snapped her out of mating lust or she just saw her opening on Patrick. And she did manage to distract the big guy long enough for Press to handle it.

So yeah, that is the big convoluted explanation I managed to pull from this turd of a movie.

Believe me, nothing is trivial. - Eric Draven, The Crow

She was never bad, she wasn't the alien from the first, she was a clone.

CG GORE IS THE WORST THING THAT HAS EVER HAPPENED TO THE HORROR GENRE!

It could be a natural instinct for the females of her species to kill their mate after procreation since Sil kills Arden after she mates with him even though he is no longer a threat to her at that point

I think they just wanted to bring Natasha back, and also wanted to pit two of the aliens against each other.

I doubt they cared much whether it made sense.