thoughtlessness

noun

1. A careless, often reckless disregard for consequences: abandon, carelessness, heedlessness. See careful/careless.
2. A lack of consideration for others' feelings: disregard, inconsiderateness, inconsideration, unthoughtfulness. See courtesy/discourtesy.

The noun has 2 meanings:

Meaning #1: the quality of failing to be considerate of others
  Synonyms: inconsideration, inconsiderateness
  Antonym: thoughtfulness (meaning #2)

Meaning #2: the trait of not thinking carefully before acting
  Synonym: unthoughtfulness