What are some good and bad impacts mechanical engineering have on society?

I agree with Calvin and Doug, good and bad is not universal. If it is manufactured, most likely an Mechanical Engineer was involved. From design of the product to the design of the equipment that is used to manufacture it. But we do have a choice of where we work and therefore the products that we design. I have worked on consumer products and medical devices. I have chosen not to work in the defense industry. With that said, I have seen the gaming controller, that I worked on, used to control military robots. In this case, it was used to determine if the field was safe to proceed.

I'm not sure I can provide a good answer to this question. As a mechanical engineer, my jobs were to ensure that mechanical equipment was designed, built, and maintained as well as it could be. Whether the equipment was used for "good" or "bad" was out of my control. I remember one time walking by an aircraft carrier in drydock and thinking that it took billions of dollars to design, build, maintain, and operate; and its main purpose was to be one of the most deadly killing machines in human history. I also remember thinking how grateful I was that we (America) had these machines, and that our main use of them was for good.

You ask a very broad question and assume that everyone's definition of good and bad will be the same as yours. Since Mechanical Engineering is such a broad field, you can be fairly sure that MEs have played a part in developing and building most of the modern World we live in today: from nuclear submarines to automobiles to airplanes to power plants to oil refineries to most of the things you use every day. If the item moves (e.g. a bicycle or an airplane) an ME was involved. If it doesn't move (e.g. a bridge or building) a Civil Engineer was involved. I suggest you Google Mechanical Engineering and what MEs do and then pick out the items that you find both good and bad.

Good Luck.