Bad

Overview
Understanding or agreeing on what is good (as well as what is bad) is obviously necessary in order to be able to evaluate the overall good or bad, or net good, of a particular decision.

Terms

 * "evil" (intending harm; deliberately bad; malicious)
 * acts whose net results can only be expected to be bad overall
 * an entity which knowingly performs such acts

Speculation
It may be enlightening to examine what types of things we, as humans, find to be positive or negative on on various levels:
 * things to which we react negatively on a visceral level
 * things which any rational person would agree are undeniably bad

List of undeniably bad things

 * abuse of the innocent: innocent people being hurt, killed, blamed wrongly
 * a special set of rules apply to children in that it is generally considered wrong to harm a child beyond specific types of (relatively mild) punishment for wrongdoing; there is, however, disagreement about the exact distinction between a child and an adult in certain circumstances, including:
 * the child is on the upper end of the "child" age-range
 * the child has performed a truly heinous act which even the very youngest should recognize as "wrong"
 * unintended harm to innocent people is also considered a bad thing
 * threats of harm, including hate speech: either the speaker actually intends to do something harmful, or they are lying (which is also considered bad, though exactly how bad depends on the circumstances)

List of arguably bad things

 * "killing" is often held to be wrong under any circumstances, or any circumstances except self-defense, but there is disagreement.