I dotn think you can be "unintentionally cruel"... It has to be something deliberate, in that you get pleassure from hurting someone....

She's certainly callous.. She does not care much at times if she causes pain, becuase she's selfish.. and she tends to think that others are more like herself, and that if they feel pain over a situation which would not hurt HER they are just being foolish and seh can't really beleive that they are very much hurt.
And Austen does IIRC tell us what happened to Mary.... but it is towards the end of hte book so Im not sure if I should metnion it