NO, PICCOLO DIDN'T JUST "THINK" HE WAS A DEMON, HE WAS ALWAYS A DEMON
I see this all the time when discussing dragon ball.
People always say Piccolo only thought he was a demon and was just a slug man.
That is wrong, that's not what happened at all. The story is very explicit that he is BOTH. He is Namekian by blood, yes, and he wasn't aware some of his powers had their origin in said blood. But in dragon ball, you can be part of the demon race or be a demon by title, just like you can ascend to godhood, and have the mystical abilities that come with that
The smoking gun is that people killed by demons can't go to the afterlife, which happened to Krillin, Roshi and Chaotzu when they were killed by King Piccolo. But Goku does go to heaven after being killed by Piccolo in the battle against Raditz, and Raditz goes to Hell.
Kami specifically comments on this, on how Goku going to heaven is a sign Piccolo might be rejecting his demonic nature after being spared by Goku
Him actually being namekian doesn't contradict this. it explains some things he thought were demonic but it doesn't make him not a demon
And yeah this is obviously a retcon but it's well written and makes sense
I know daima then retconned it so all namekians come from the demon realm so I guess this doesn't matter