A bad thing is something or someone whose repute is entirely negative. Vandals are almost always thought to be a bad thing. So are trolls (by the sysop power structure that is unable to admit that it is guilty of sysop vandalism), but troll organizations and Trollism have emerged to challenge that claim.
It may be valuable to discover whether something is universally claimed to be a bad thing in a large public wiki. Finding things that actually have some defenders that are claimed to be bad things is an indication of systemic bias and possibly a persistent Sysop Vandal point of view.