Revelation 6:9-11 really bothers me...

I keep running into this passage that has always bothered me. It is from Revelation 6:9-11

When he broke open the fifth seal, I saw underneath the altar the souls of those who had been slaughtered because of the witness they bore to the word of God.

They cried out in a loud voice, “How long will it be, holy and true master, before you sit in judgment and avenge our blood on the inhabitants of the earth?”

Each of them was given a white robe, and they were told to be patient a little while longer until the number was filled of their fellow servants and brothers who were going to be killed as they had been.

I'm trying to figure out what is going on here. This sure sounds like people who are martyrs. These are people who I would think of as saints... who above anyone else should be enjoying the beatific vision and paradise. Instead, they seem to be gathered in a separate place, literally under a sacrificial altar, and clearly perturbed about the lack of justice they have experienced not only in life, but now in the afterlife! I'm wondering how others might be interpreting this, and what the meaning is of the white robes.

I had heard that this was actually a representation of purgatory, but it sure doesn't make sense based on the people having been "slaughtered" so I don't really buy that. I know Revelation can be hard to understand, but what do other people make of this?