Action is out of a conscious decision. Reaction is out of impulsiveness.
Impulsiveness creates a chain of Karma.
Reaction, as well as non-action, both create karma, but a conscious action transcends karma. Conscious action does not create new karma whereas non-action can create karma.
For example, a soldier shooting in war does not create a karma; a policeman using tear gas does not create a Karma, and if you are a doctor and you do not give medicine to a patient in need, you are incurring a karma.
Through knowledge and devotion, transcend all karmas and be free!