It’s not that the fandom made Bill Cipher ‘edgier,’ he was already pretty dark. They just make him less funny (sometimes that involves being less G-rated but ultimately he’s portrayed as less of a goofball in fandom and more as a source of horror and nightmare fuel).
Like, don’t get me wrong, Bill is definitely a terrifying monster and a power-hungry sociopath with no morals, but he’s also basically a shitposter. Most of the things he does are like something spawned from the mind of a sleep-deprived funnyman with a really sick sense of humor…probably because they are.
“What’s a good thank you gift for a kid? Teeth. Just, a whole pile of deer teeth, fresh from the deer. Kids like teeth, right?”
“If this fucker tries to make a deal with me, I’m putting an eye where his mouth is and putting noses on his eye sockets. Try eating that pastrami now, bitch,”
“The Earth…tasty. Hot gooey center. Love the cronch,”
“Humans are basically puppets. I get inside them and control their bodies and slam their arms in a drawer” “I think you horribly misunderstand what puppets are for, Bill,”