Tomie is exactly as you said, she is a means to bring out the worst in people.
But I believe she is her own character once you tracked her growth from her initial introduction up to the later editions to the series, things changed. She starts as a victim seeking revenge and later on develops a character that becomes a symbolic evil with each edition of the story.

I think this was a crafty move by Junji when he serialized the series. It's good if you read only one work on Tomie but if you piece them all together, Tomie's personality with each death and it's a subtle build up. The Tomie from the early works had reasonable grudges while the Tomie from the later works seemed like evil as a phenomenon rather than human with a grudge.