Why would you be evil just because you're mistaken?
Why would you be liars just because you're mistaken?
The best "mythicist" position that I know of is the one that is advanced in Richard Carrier's 2014 OHJ, but to my knowledge he never says that there is any deception/lying going on.
He does talk about how early Christians wanted to historicize Jesus in order to fill the pews and delegitimize other Christian sects, but to my knowledge the claim is that these Christians all genuinely believed that Jesus had really existed and died for our sins and everything in the lower heavens...
It's frustrating that nobody reads OHJ. It's a much more serious challenge to historicity than some simplistic conspiracy-theory that you read online. It's the book that you want to engage with if you want to defend historicity; it presents the best mythicist position out there.
See here where Carrier explains that Paul (at least) did believe Jesus was real but that he believed that Jesus existed in the lower heavens:
https://www.youtube.com/watch?v=58dG-wWBPBI&t=1200.