misconceptions or contradictions? in one sentence you state with conviction there is no heaven or hell, only for your next sentence to say you cannot say it with 100% certainty, just to loop back to there being no heaven or hell as it is merely a creation of the church [and by that logic Christians are liars not a group with different spiritual views] to finally conclude it's just your opinion on the matter. pick one.
is heaven and hell real? it's impossible to say as these realms are the result of faith and the religion that preaches these places it is 100% real to their belief. if you follow a path that does not believe in a land of eternal reward/punishment after death based on your life then no, it does not exist in your belief. there is no solid answer when it's personal belief. you can only state your personal views with conviction.
can you become a demon after death based primarily on your opinion bad deeds result in demonic reincarnation? yes and no. again, there is no solid evidence of what happens after we die [regarding the soul anyway] but demons [that is to say demon mythology] have been studied for a long time, and i'm not aware of any that came about a person doing a certain number of evil deeds. but if you believe truly wicked people are reborn demons then perhaps they can be. personally, the definition of demons i know is souls that were never human, but the theory evil souls become demons makes some sense. tales of how demons came to be are rather vast, so maybe there is a way to do enough 'evil' to become a demon [of course, evil is societal, so if society deems lying, theft and murder 'evil' you can as well, but you could always argue a way for those things to be good. protection for example, to help your starving family for another.]