Are all bacteria bad?

No. Not all bacteria are bad; many are important some of the body's most basic processes, including digestion. Bacteria is also a necessary in the formation of certain foods such as cheese. Lastly, we need the normal flora bacteria to compete against the "bad" bacteria in our body, preventing the bad bacteria from overtaking our body.
No. Our bodies are full of bacteria and most are note bad at all. Fecal mater has hundreds of thousands of bacteria, all of which serve a purpose. However, even a "good bacteria" in the wrong site can cause problems.
No. Many bacteria are helpful, including ones that live in our intestines and help the digestive process and keep out more dangerous bacteria. Also, bacteria are largely responsible for decomposing waste in the environment, ultimately returning complex things into elementals that can be used (such as soil nutrients). Bacteria are everywhere, but only a few species are really harmful.