I'd say it depends on what the intentions are:
If you do not intend to kill any ants/snails or slugs, but do so unintentionally just walking or something - I'd call that an accident as with any other animal killed this way and hardly something to be fined for or animal cruelty.
As for other intentions such as trying to keep slugs from eating up your garden, making sure cows don't eat them and get sick as well as keeping ants from invading your home etc., that is, I'd say, a good reason to invest in pesticide or whatever way of controlling this.
As far as stepping on ants, doing whatever to slugs or snails be it using salt or some other method, just to kill or torture them, that IS animal cruelty and sure, why not? Fine them, arrest them depending on the severity.