These are halcyon days for conspiracy theorists. It's possible that with the help of the internet such theories will propagate exponentially forever and we'll look back on 2016 as a more innocent time, but I don't relish the opportunity to live in a world where a wider range of conspiracy theories are more gleefully embraced than they are today.
Conspiracy theories are appealing for two reasons: they make the world seem more interesting and exciting than it really is ("Hillary stole the nomination" is a much better hook than "It was unlikely that a 74 year old Vermont socialist who isn't a member of the Democratic Party was looking at an uphill battle to win the Democratic nomination, and honestly if there was a conspiracy against him he wouldn't have come nearly as close to winning as he did") and they allow people to shift responsibility. We didn't lose, we were cheated. I'm not a failure, the (Jews, Unions, Liberals, feminists, immigrants) stole what I deserve. Both of these things make conspiracy theories inherently appealing, and for valid reasons. Nobody likes being bored or having to blame themselves for the things they aren't happy about.
Not all conspiracy theories enjoy equal acceptance, though, and the most plausible ones – the ones that seem like they have enough circumstantial evidence to make them true-ish – have the upper hand. You don't hear anyone but the most fringe unstable types talking about chemtrails, but large numbers of people believe, for example, that global warming or the price of gasoline are the products of conspiracies. Identifying a group of people who could benefit (Scientists! Environmentalists! Liberals!) a theory seems more plausible, and identifying individuals or groups who seem to have control over a fluctuating commodity (OPEC! Obama! The Saudis! The Jews, because it's always Jews!) allows people who don't care to bother with the details below the surface to take comfort in having found the answer.
I do not for a moment endorse the conspiracy theory that Trump is tanking this election on purpose to ensure that his supposed close buddy Hillary Clinton wins – Look, they stood next to each other in a picture in 1994, what more evidence to you need? What other explanation could there be for the First Lady and a famous rich person to have been in the same room together? – but the past three weeks have made it perfectly clear why so many people have latched onto it. I don't believe it, because there is no evidence to support it. But Trump has melted down so completely and seems to be going so far out of his way to think of what he can say that will finally get his acolytes to stop cheering for him that it's only logical for people who are cynical and pay limited attention to politics to seize upon this as an explanation for his otherwise inexplicable behavior. Trying to make sense out of nonsense is normal.
The truth, as usual, is more mundane: the man has a personality disorder, and an attention-craving narcissist who relies on shock value has to continually up the ante in order to keep achieving the same effect (think Howard Stern, Marilyn Manson, etc.) And once you've attacked a dead US Army soldier's parents and asked Russian agents to steal classified information to help your campaign, I guess the only way to increase the shock value from there is to make "jokes" about your supporters killing Hillary Clinton. That's why it seems like this is playing out to a script, like each week brings a calculated increase in the extent to which Trump seemingly goes out of his way to get people not to vote for him.
I don't believe Trump cares about anyone but Trump, or that he is trying to help Clinton win. I do understand how people could look at this shitshow and come to that conclusion, though. The popularity of that theory is the best testament to just how bad he and his campaign are. We're witnessing something historic here. Think of how badly a team would have to lose the Super Bowl, for example, before it would enter your mind (and seem plausible) that they were losing on purpose to help their friends on the other sideline.