Sugar is sweet, and there's nothing wrong with being sweet. Sweet is non-threatening and kind and happy.
There's not a lot of it about.
Saccharine, however, is fake sweet; and that's revolting and dangerous.
You get saccharine greetings cards verses (I could give you an example, but don't worry, I won't). You get saccharine bad art.
You even get it, sometimes, in art that's lauded and pricey. The implied irony is supposed to make up for the insincerity (though it almost never does).
Sodium saccharine, or benzoic sulphimide is hundreds of times sweeter than sugar, but has a nasty after-taste. This after-taste, however, can be disguised by other chemicals, and the stuff is used in all sorts of low-sugar food products.
Sodium saccharine gives rats bladder cancer, but humans cope with it well.
With the simpering and gushing kind of saccharine, though, it's usually the other way round.
Thing Not To Be Today: saccharine. This word comes from the Latin saccharon, sugar. Before that it comes from the Sanskrit sarkara, which means grit or gravel.