General Negativity:
* Awful
* Terrible
* Horrible
* Dreadful
* Nasty
* Poor
* Shocking
* Unpleasant
* Unsatisfactory
* Disastrous
Negative Qualities:
* Wicked
* Evil
* Corrupt
* Defective
* Flawed
* Faulty
* Inferior
* Substandard
Negative Actions:
* Wrong
* Improper
* Immoral
* Illegal
* Harmful
* Destructive
* Unacceptable
* Unforgivable
Feeling Negative:
* Sick
* Ill
* Unwell
* Miserable
* Depressed
* Down
* Blue
Informal:
* Lousy
* Rotten
* Crummy
* Stinky
* Yucky
* No good
Remember, the best synonym for "bad" will depend on the specific meaning you are trying to convey.