It seemed to me that the things Rizzo sings about that she considers "worse" than her behavior are how she views Sandy to be and maybe some of that hits home for Sandy.
I could hurt someone like me
Out of spite or jealousy.
I don't steal and I don't lie,
But I can feel and I can cry.
A fact I'll bet you never knew.
But to cry in front of you,
That's the worst thing I could do.
This more than anything sheds a whole new light on Sandy's impression of Rizzo. Rizzo just admitted she has feelings just like Sandy and maybe they're not so different after all. Perhaps Sandy really is no better than Rizzo and perhaps Rizzo is no less deserving of respect than Sandy. Learning this, perhaps Sandy doesn't think being a member of the gang is such a bad thing, after all.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian