Here is the a quote from Art's wiki entry
Beginning in late 1996, Bell was criticized for reporting rumors that Hale-Bopp comet was being trailed by a UFO. It was speculated that members of the Heaven's Gate group committed mass suicide based on rumors Bell aired, but others dismissed the idea, noting that the Heaven's Gate website stated: "Whether Hale-Bopp has a "companion" or not is irrelevant from our perspective." Susan Wright notes, however, that Bell was also "one of the first to publicize expert opinions refuting the 'alien companion'" said to have been shadowing Hale-Bopp.
I agree that Art was complicit in propagating the UFO rumors which was stupid on his part, but I don't believe Art had any complicity in the deaths of the Heaven's Gate cult. Compared to most of the doomsday crap spouted by talk show hosts like Alex Jones, Glen Beck, etc., The UFO rumor is pretty mild. Heaven's Gate would have off'ed themselves somehow...
Here is another wiki excerpt about the incident from the Comet Hale-Bopp entry
In November 1996 amateur astronomer Chuck Shramek of Houston, Texas took a CCD image of the comet, which showed a fuzzy, slightly elongated object nearby. When his computer sky-viewing program did not identify the star, Shramek called the Art Bell radio program Coast to Coast AM to announce that he had discovered a "Saturn-like object" following Hale–Bopp. UFO enthusiasts, such as remote viewing proponent Courtney Brown, soon concluded that there was an alien spacecraft following the comet. Several astronomers, including Alan Hale, claimed the object was simply an 8.5-magnitude star, SAO141894, which did not appear on Shramek's computer program because the user preferences were set incorrectly. Later, Art Bell even claimed to have obtained an image of the object from an anonymous astrophysicist who was about to confirm its discovery. However, astronomers Olivier Hainaut and David J. Tholen of the University of Hawaii stated that the alleged photo was an altered copy of one of their own comet images.
A few months later, in March 1997, the cult Heaven's Gate committed mass suicide with the intention of teleporting to a spaceship they believed was following the comet.
Nancy Lieder, a self-proclaimed contactee who claims to receive messages from aliens through an implant in her brain, stated that Hale–Bopp was a fiction designed to distract the population from the coming arrival of "Nibiru" or "Planet X", a giant planet whose close passage would disrupt the Earth's rotation, causing global cataclysm. Although Lieder's original date for the apocalypse, May 2003, has now passed, the imminent arrival of Nibiru is still predicted by various conspiracy websites, most of whom tie it to the 2012 phenomenon.