Why do they eat everything

Because they are hungry.

and where do they come from?

God made them - ask Him

How often must they multiply??!

There is no "must" associated with their reproductive habits - they eat,
they poop, and they multiply. The latter two are probably somewhat
proportional to how much they do the first.

What do they do? besides eat my garden and ooze slime!

Eat. Poop. Multiply. Ooze slime.

Is pouring salt on them cruel? do they know they are dying or am I
causing other harm by doing this?

That is a very good question! Are they self-aware? Are they automatons? Can
they even feel pain? Are they no more conscious then a computer program? A
computer can be programmed to give responses that indicate sorry, pain,
etc., but it has no self-awareness that can suffer. Are slugs any better
then that? I doubt anyone knows.

Help! Ive tried everything else!

Turn off your computer and go outside and play. And watch out for the slugs