Gargoyles?

I have been in Europe and have seen stacks of old churches. One of the things that puzzles me is that these churches and cathedrals are often ornamented with horrible, evil looking and grotesque gargoyles. I kept wondering "Why? - What is their significance?" I googled it and Wiki told me they had a very practical reason. When it rained the water ran through them and away from the building so that it did not damage the mortar. They were in lieu of what we plumbers call "downpipes". The water from the roof flowed through them, out their open mouths (often extended by flames or an evil looking tongue) and away from the building.

I accept that, but why such evil looking beasts? I would think that Churches were scary enough places without these nasty beasties adorning them. Why not have smiling happy people? Most look like nasty demons. I think also that often they are just ornamental and are not the down pipes. The nasty ones seem to just be on Churches.

Are there any historians out there who can tell me the reason for these nasty looking additions to ancient Church buildings?