Similar to the phrase death on a stick, Satan on a bun refers to anything on a bun (sandwiches, burgers, ect) that looks incredibly menacing. It does not necessarily mean that the food can kill you. For example, a Big Mac, though extremely disgusting, could not be considered Satan on a bun because it is not a large, hard core, evil piece of food. The creation of the phrase can be credited to Sam Lindberg, who used it to describe a massive sloppy joe.
"Is that the sloppy joe I ordered? That thing's huge! It looks more like Satan on a bun!"
"I ordered a 24x24 at In and Out Burger... but what I got was Satan on a bun..."