Alyona is the pawnbroker Raskolnikov murders. Like Pyotr Petrovitch Luzhin, she is a flat character. We can sympathize with her because she is a woman in her late sixties, because she is helpless before Raskolnikov, and because she is killed in a brutal and cruel way.
On the other hand, we know she cheats poor people and beats Lizaveta. Still, we probably wouldn't sentence her to death for those acts. She's a brilliant choice of victim, from a fictional standpoint – a cruel person, but probably not one of the cruelest people in St. Petersburg.
Because she is so helpless, so unequal to Raskolnikov in strength, it makes his crime harder to for us to understand than, say, if he'd killed somebody that had a chance against him. By making his crime against her so brutal, so cruel, we have to think harder about whether he is a good person whom we can trust.