A sin means an action which hurts any living being physically, mentally and obstructing spiritual growth. Action means not only physical but also verbal or even by thought.
According to Hindu scriptures there is "Thrikarana Shudhi. It means our thoughts, words and actions should be pure. Why because every action has its equal reaction. Its result comes to us in ten folds .Bad action brings bad result while good action brings good result. What we are now is the result of what we did  either in the present or past lives.
Thought is also an action. How? The thing that is going on now is the physical form of a thought. Thought has vibrations. It becomes into an action with the help of cosmic energy.
So it is safe to not only to others but also ourselves to be pure in thinking, talking and acting.