We don't really understand any group, yet alone the actions of a group that commits suicide. But if we assume that Heaven's Gate was a group like any other, then we can speculate about the factors that may have led it to make such a dreadful decision. When people must make important decisions, they turn to groups.
Groups can draw on more resources than a lone individual.
Groups can also generate more ideas and possible solutions by discussing the problem. Groups can also pressure individual members to accept the solution, even if they have doubts. People generally feel that a group's decision will be superior to an individual's decisions. Groups, however, don't always make good decisions. Juries sometimes render verdicts that run counter to the evidence presented.