There are conspiracy theories and there are carefully considered theories about events that suggest a conspiracy. Adam Smith was rather good on how small groups of people can cohere to conspire against the public good in their own interest. Most of our public life is a complex interconnecting set of such small-scale conspiracies.
As to conspiracy theories, we should remind ourselves of their function and why they emerge. Like grimoires in Early Modern Europe, they are texts and ideas that explain the world to the uneducated but the uneducated cannot be blamed for this. They are left in the dark, given half-truthful narratives that they are intelligent enough to distrust.
Conspiracy theories emerge because there is a vacuum in social truth-telling and they fill that vacuum. The vacuum is created by those in power or dependent on power developing a theory for that hold on power that just does not fit as the 'whole truth'. The cracks show (Watergate, Gulf of Tonkin, whatever) and the official theory looks dodgy.
The entire of our society is built on a contestation of half-truthful narratives, stories we tell ourselves to justify our situation or our hold on power or our lack of power. If the 'deplorables' choose to fill a vacuum with their own tales, it is only because our tale lacks credibility for them and often should lack credibility for us.
The current crisis over the Salisbury nerve agent is typical. The Prime Minister jumps in with assertions and propagandists create narratives (on all sides) that are clearly a 'story' designed to get people behind particular tales of perfidy, She may be right but the assertions are premature. Alternative explanations are dismissed.
The lack of intellectual integrity in exploring the complexities of Russian state control of its assets past and present and the motivations and interests of the many parties involved and the opportunity to tell the public the facts without assumptions until investigation is complete is lost and creates the space for surmise and distrust.
From this perspective, a conspiracy theory is simply an act of defiance where you create your own falsehood as a story you chose over the incredible tales of those who are determined on not handling an issue with transparency.