Although I also sort of agree with the people who pointed out that Life was the Big Bad, however, I think it was just A big bad, just a theme throughout the season. Because, eventually, everything led to Willow in the end: the magic abuse, Tara leaving, Warren killing Tara and the general miserableness of everyone, which she couldn't handle. So life was bad, because it led to Willow becoming the real Big Bad. Or, that's at least how I view it.