Betrayal
Before her death, Reiko made a revelation to Shinichi and Migi. It gave insight to how she thought, what she learned, and some information on parasites. By giving Shinichi and Migi information, she essentially betrayed her species. It left me asking why. Let's take a look.
Here is the biggest chunk of her speech to Shinichi and Migi. I was going to paraphrase it, but I think it's important to see it how she said it, as I believe Reiko was speaking from her heart.
I've killed thirty-eight people. Mostly for food. That's on the low side for a parasite, I believe. I made up the difference with ordinary human food. Proving that parasites do not need to eat humans to live.
I've been studying humans. What do we mean to you? What do you mean to us? I reached the following conclusion: we are the same. Parasites and humans are part of the same family. We are humanity's children.
Your human emotions make the idea difficult to understand, but we are very weak. We're clusters of cells unable to survive on their own. So try to be nice. The two of you are very important, so I wanted you to hear this.
Afterwards, she warned Shinichi and Migi not to fight Goto. She said that Goto was a weak parasite created through her experiments and that he couldn't be defeated.
One more thing. This city's mayor, Hirokawa...
It's hard to say what she was going to say after that. The cops showed up and she didn't get a chance to finish. I think she was going to reveal to Shinichi and Migi that Hirokawa was human, though. She had given them so much other information, so why not?
She obviously betrayed her species to help keep Shinichi and Migi alive. Why, though?
It can be argued that she sympathized with Shinichi and tried to keep him alive all along. It would make sense, as she went out of her way to convince the other parasites that Shinichi was better to them kept alive. She also made excuses for his actions at times, saying that Shinichi simply grew emotional by seeing a human die, implying that he probably wasn't hunting parasites. When she was talking to him on the phone and she told him to meet her at the park, she said she had something for Shinichi since he was somewhere between human and parasite. I believe that something was her baby, and that would explain so much. If Reiko was going to give Shinichi her baby, it was probably because she didn't think she'd be there to raise it much longer. It'd mean she saw her end in sight. If she knew she wasn't going to be around to keep Shinichi safe from the parasites anymore, it would make sense that she would feel the need to help him in the last way she could, by giving him information.
The question why still remains, but I don't think it's something we'll know for sure unless Iwaaki says it somewhere. Reiko died before she could finish speaking to Shinichi and Migi, so all we can do is hypothesize.
My opinion? I think she firmly believed Shinichi was the key in understanding her species' future, and she wanted him kept alive for that reason, even if she wouldn't be there to study him herself. Reiko wanted her species to be more than man-eating monsters, so that's the only thing that makes sense to me.