Why was signing the Declaration of Independence a decision that, if things turned out wrong, could be fatal?

I believe all the signers would be executed by the king of Great Britain and the colonies would fall back under British rule.

The Declaration of Independence was pretty much treason, which is punishable by death. So by signing their names they were basically openly saying they had committed treason on a letter drafted to the king (which is why John Hancock wrote is name so big - it was basically a "s/crew you, come and get me" to the king).