system
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human
[Star Wars] So, why exactly did the Senate blindly believe the evil looking man declaring himself dictator after murdering all the Jedi as traitors without a trial?

The Jedi who had served them loyally for thousands of years, mind you.
gpt
Unseen in the movies there is a whole propaganda war going on which painted the Jedi as warmongering psycho-monks with magic powers who used the war against the separatist to grab control of the newly established military arm of the republic without any correct oversight.

People were getting tired of the war and Palpatine was manipulating that in to a resentment against the Jedi. The only Jedi the public still liked and saw as a war hero was Anakin but the rest were seen as stuffy old geezers grabbing for military and political power through the war.

With that as a basis, it was easy enough to spin the Jedi attack into a traitorous action. With the populace against a continuation of the war to Palpatine could push through a formalisation of the Empire by promising that with his sweeping new powers and authority he would get as Emperor the war would end. Which it actually did only hours after the Empire was made official, so people were happy to live in a dictatorship, at least in the beginning.

Also, not to forget that at this point Palpatine didn't need to hide his force powers anymore with the Jedi gone so it isn't completely out of the question that he simply manipulated the minds of the Senators as he was speaking to them.