I don't wish to get tangled up in the ever present and long-suffering argument about what others have termed fluffy-bunny wiccans, lightside pagans and whitelighters - however - I do believe that many of the newcomers to Wicca and Paganism are leaving out an important part of the religion. So, as usual...
Wicca is as much a way of life as it is a religion, and one of the important concepts of this , or any pagan, religion is balance - in everything. For one to achieve such balance, we have to acknowledge not only the positive and light sides of our deities, but also the dark and sometimes negative.
We've all heard it, we've all said it - "Wiccans (and generally Pagans) have no concept of Satan or Evil". Well, we don't - or most don't, anyway. Light and Dark, Yin and Yang, positive and negative, these are things that abound in nature.
The darker aspects of the God and Goddess are generally deemed as the most mysterious, dangerous and violent. Kali, a popular Hindu goddess is known as the Mother of Destruction. Having just seen that moniker - and doing no further study - it would be easy to assume that she is a violent, ill-tempered, and just down right nasty. According to Hindu mythology however, this is quite untrue - she "...destroys ignorance, that which incurs chaos or disrupts harmony, the ego which interferes with the work of God, and blesses those who strive to know God." This is not to say that anyone should ever think of Kali as a Goddess of sweetness and light, however, we can discuss that more later.
The dark Goddess and Horned God are the counterpoint to their life giving, brighter, aspects we normally associate with on a daily basis. They bring death - death is a precursor to life. They keep the 'secrets' of magic and mystery within - without hidden mysteries and truths our life-long journey as humans wouldn't have much purpose. They are destroyers - without destroying what's no longer needed, new things have no where to grow.
Darkness is so often associated with evil, that those new to Paganism and Wicca often find it difficult to reach beyond the stereotypes - which have been ingrained within them for many years. Many in the "New Age" movement believe that if you "get in touch with Nature" your life will be all happiness and joy.
Nature, however, is as harsh and unforgiving as is it beautiful and nurturing. The darker aspects aren't all death and destruction, though. Autumn and winter are many peoples favorite season, wisdom and maturity are sought after by everyone and being reflective or introspective never was a bad thing. What do all of these things have in common? They are all associated with the Dark Goddess, the Crone aspect and the Horned God.
Is night considered evil because its "killed" the day? When day is reborn in a beautiful dawn, it is, after all killing the darkness, in a manner of speaking.
This, and many other cycles of everyday life, remind us of the most important cycle, the unending circle of life, death and rebirth.
There is much more to be discussed about the darker aspects of the god/desses of the world, but this gives you a general idea. I believe one cannot live a balanced life while only acknowledging the "light" side of life. However, to each their own!