Lizaveta is the woman Raskolnikov didn't intend to kill...which is only slightly better, ultimately, than being the woman he did intend to kill. Well, actually, there's no appreciable difference: they both end up deader than disco (and deader than the expression "deader than disco").
In fact, part of why he feels justified in killing Alyona, Lizaveta's half sister, is because she beats and exploits Lizaveta. Lizaveta is a 35-year-old woman who appears to be mentally challenged, is at least 6 feet tall, is incredibly shy and meek, and is often "with child" (though we never hear about her kids).
Yup. That's pretty much all we know about her.
In Raskolnikov's mind (at least before he starts getting axe happy), Lizaveta, Dounia, and Sonia have almost merged into a single abused person whom he feels completely powerless to help. Thinking that Lizaveta won't be at home when he plans to kill Alyona is one thing that motivates him to follow through with his plan.
The irony is richer than Bill Gates. If he hadn't been running late, if he hadn't forgotten to shut the door, and if he hadn't reacted to Lizaveta's presence by lowering his axe, she wouldn't be dead...and just maybe his plan would have worked. Yet, until Raskolnikov and Sonia get together, Lizaveta doesn't seem to weigh much on his mind, or anybody else's.
When Raskolnikov learns that she and Sonia were friends, he starts to think about Lizaveta a little more. When he confesses to Sonia, he confesses to Lizaveta's murder. Similar to the way Raskolnikov really sees Sonia's value when the prison community accepts her, he really sees Lizaveta's value when he sees that Sonia loved her.
Yeah. No one ever accused Raskolnikov of being the world's best judge of character.