Odo is a bad choice (apparently that was the sole purpose of the dungeon scene, since he's been relatively decent and clever otherwise) but Rollo's character is mixed. Besides the slave rape, at various times he's also betrayed his brother, been a drunk and frequently not treated Siggy with respect.
I have to had it to the show though. As much as I love many of the characters, I still almost never root for the Vikings. Again last night I was firmly on the side of the French. Most of my favorite characters are Vikings, yet I'm not on their side. I don't think there are many shows that can pull this off as well as this show has done.