Hi - and welcome !
Any non-native species that is released can cause an imbalance in the eco-system. Species evolve side by side, and when a non-native species is introduced, even with the best of intentions things sometimes run amok. One example I can think of is the tilapia. Tilapia are south american and are herbivores. At one time it was thought that introducing them to lakes in north america would help to control "unsightly" plant matter. They did this to the lake behind my house. What they didn't realize was that the tilapia had no natural predators here (as they do in the wild) and would actually eat TOO MUCH plant matter - and now the bass that once were abundant in the lake are becoming hard to find
You can find lots of tilapia, but you can't fish them effectively since they don't take bait. Oddly enough, even tho they are now considered a nuisance fish you need a special permit to take them (the laws haven't caught up yet !).
Another example are European Starlings. These birds were introduced accidentally - they have now outcompeted our native american species, so our own native populations are in decline, while the starlings are spreading across the country rapidly.
Introduced snails are now causing similar issues. Depending upon the species they may have no natural predators, which means they can overtake the ponds and lakes, which would throw the eco-system out of balance and reduce the native species.
(Sorry - I'm not lecturing - you just happened to ask a question that I know a bit about because of volunteer work I had done for several years and a topic which I have pretty strong feelings about - LOL, like I had to tell you that).
I don't know how to humanely euthanize snails, but I would imagine that the ice-water method would work on them. Basically you fill a glass with water and ice and then place the animal in it. The shock generally kills them. There's also a "clove oil" method, but I'm not sure if it would work on snails
http://www.wisegeek.com/what-is-the-most-humane-way-to-euthanize-a-fish.htm