Actually, RK, while Tesla did do work on DC while he was employed by Thomas Edison, it was actually his work on AC that he felt was his important contribution at the time. Edison was having none of that, however, and Tesla went to work for George Westinghouse, where he pretty much perfected the basics of polyphase AC generatoprs and motors, fluorescent lighting, and much more.
Edison, in fact, was DC's primary promoter, vs. Tesla's and Westinghouse's AC system. Tesla was generally pretty much dismissive of DC.
The whole elephant thing was Edison trying to prove how dangerous AC was, since AC's economic advantages (longer transmission distance and thinner wires needed to carry equivalent loads) threatened his business. He certainly advertsed the "advantages" of electrocution as a means of execution but probably went a bit too far when he suggested that the procedure should be called "Westinghousing".
The first electric chair was officially (and probably actually) designed and built by the electrician at Auburn Prison in New York although Edison MIGHT have had a hand in it, since he was involved in the subterfuge needed to buy the necessary generators from Westinghouse (who refused to sell them to the prison, for obvious reasons).