The Minority Report scenario
The scenario you have there is a variant of what we have in the film Minority Report. This scene touches upon the problem but does not go the full length with the argument.
Before we go into that, let us re-iterate some fundamentals of criminal law:
- You can be sentenced for actions that you have committed
- You can be sentenced for actions you were just about to commit, and only fortunate circumstances prevented you from going through with them
- You can be sentenced for the action of preparing to commit a criminal action, although this is a lesser crime
- You can not be sentenced if there never was any chance the action you were about to commit could have succeeded
- You can not be sentenced for any actions you have not committed or attempted to commit
In the Minority Report case they dodge that whole problem of not being able to sentence people for things they have not yet done in that precognition happens so late that the law enforcement are always arriving in the nick of time; the crime is well underway and it is only by their intervention that the crime is stopped. So in Minority Report, they can sentence people for this: "we are sure that if it was not for us you would have gone through with it".( * )
Your case however is different. The murder is not even close to happening yet. Everyone besides (B)ob knows about it so they can whisk Bob away well ahead of time. And everyone can tell (A)lice "Hey, we know... do not be trying any funny stuff now!".
Then again... Alice has already gone through with the action. Alice actually did it. The fact that the damage has been undone should not matter. The action has been performed, so it should be actionable.
Then, yet again: where is the proof? Bob is alive, so there is no dead body. There are no forensics. There is no evidence. And even if the entire world knows that — yes — we have had a magic time-travel event( ** ), all they have to convict Alice on is the sworn testimony of those that started the investigation. And Alice can lawyer the living daylight out of the prosecution:
- "Nope, was not me, you cannot prove it was me"
- "Ok, so I did it but it was self-defense"
- "Fine, it was not self-defense but you still cannot call it murder, it was manslaughter or an accident.
- "Bah... it was no accident but I was mentally incompetent at the time due to depression/schizophrenia/being extremely upset because Bob did something really nasty to me and mine. I am fine again thanks to the magic time-travel event."
There is no way to get a case that will stick to Alice, hence they cannot possibly convict her.
So in the end...
...it probably turns into one big "Meh...". Alice is released by the authorities with an annoyed grumble, a warning finger pointed in Alice's face saying "Don't even think about doing it again!", and a huge sigh of relief that Bob is OK.
( * ) SPOILER: And then there are some other flaws in the system, but those are a separate issue.
( ** ) Or have we really? Maybe someone just screwed with our memories and implanted false memories of a potential future?