I know that it was Light's plan to make Misa look suspicious again so that Rem would kill L. But why did she think that Misa was going to be executed?
Was it because her lifespan was shortened again?
I thought that only a Death Note can shorten the lifespan of a person, and even though she did the Shinigami eye deal again, was her lifespan too shortened?
Or can your lifespan get shortened by other people, because of their actions?
Basically, L told her what would happen, when she would be caught. Light made it pretty obvious that Misa was the new Killer, letting all evidence point in her direction. He made her kill people the instant she left the mansion. Especially since Misa was the original suspect for being the second killer. If it wasn't for the fake rule, Misa would have been arrested or at least thoroughly investigated and L had doubts about that rule from the beginning anyway.
L:
If the murderer by notebook is recognized, at the very least, if we cannot get the capital punishment, we would have them write their own name in the book. That's how it is. chapter 57
Rem, being pretty sharp herself, came to the same conclusion. She knew L would figure it out, she knew it would mean the end for Misa. Though, she had made a promise to protect her till the end, so she knew she had to kill L to prevent Misa from dying early.
Rem:
The one who is killing the convicts right now is Misa. There is no doubt about that. Since the notebook has been revealed, no matter what happens from this point on, the one who will be caught as Kira will be Misa. That wont change...
Yagami Light is confident that I will save Misa's life... The only way to save Misa in a situation like that would be for me to write Ryuuzaki's true name in the notebook...
And in that situation if I were to kill Ryuuzaki then I would have clearly meddled with Misa's lifespan and die. chapter 57
This is for me the weakest play from any character in the series. I've searched through the internet to see if someone had the same thought that I did but haven't found any mention to it.
Rem wants to protect Misa
Rem needs to be alive to protect Misa
Then, Rem can't protect Misa if she's dead
With this simple point in mind, what she did was only the last resource she had, a resource that she should of have taken after discarding other ways of action. If she's dead nothing stops Light from killing Misa on his own (which was his initial plan).
Why didn't she talk to L and negotiate? Give him two options:
Option 1 (what she partially did): I kill you, Watari and the whole task force and you'll never catch Kira nor Misa, failing in every aspect of your investigation.
Option 2 (what she could have done before going for Option 1): I tell him that I can see who's the first Kira, and that the second Kira is Misa. But that as a Shinigami who's involved with Misa, I will not let anything happen to her, so I can either kill them all, or give them, in exchange for Misa's innocence), the name for the first Kira. If they refuse this deal, then they will have none (and go to Option 1).
There's a chance L would of rejected this deal, for an ideal about finding through himself who's Kira or an ideal of justice where he wouldn't want Misa to have immunity. But in the face of death, there was a chance where he could see that one of the two options had a better outcome than the other.
The police suspected Misa of being Kira, and she was about to be exposed. Rem realized that Light had purposely set Misa up to get caught so that the only way to save her would be to kill L, otherwise, Misa would get the death penalty for being Kira. If a Shinigami helps a human by extending their life like this, the Shinigami dies. Even knowing this, Rem sacrificed herself in order to save Misa by killing L and making it so Misa would no longer be a suspect.
Also see: What was the intent for making Rem sacrifice herself for Misa?
to save miss I think. because if watari told L the information he had, then both Light and Misa would be executed.
