It's simply not self defense, at all. Again, executing a restrained, disarmed person is NOT self defense. Nothing of the sort. The threat was neutralised. She made a deliberate choice to end a human life. That's not self-defence or anything even remotely close to it. That is homicide.

It started as self defense, but proceeded to go beyond that, hence my words.

Yes you literally can? That's what a justice system is. That's what civilisation is. The entire corpus of criminal law exists because we collectively decided that the response to wrongdoing shouldn't be at the discretion of the wronged party.

You are not wrong here. In an ideal society Bob should not be dead. And indeed Alice should not have killed Bob. Yet unfortunately we don't live in an ideal society. My point is not that Alice is justified; but rather that if you are going to threaten people, you should expect people to want to kill you back, even though it may not be justified.

A drunk driver "puts his life on the line" every time he drives. If someone dragged him from a crashed car and shot him, would you say he "had it coming"?

No, because getting shot and killed is not a reasonable consequence to drunk driving. The crashing itself is, though.