Lizaveta is the woman Raskolnikov didn't intend to kill. 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 thirty-five year old woman who appears to be mentally challenged, is at least six feet high, is incredibly shy and meek, and is often "with child" (though we never hear about her children or anything like that).
Before the murders, in Raskolnikov's mind, 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 rich. If he hadn't been running late, if he hadn't forgotten to shut the door, 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 same 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.