The Mutant Massacre was THE event back in the 80's which made darkness and death the stars but it was done right and lasted only an arc. The following decades made strides into walking away from the doom and gloom, and expanding the X-stories, making characters grow up, seeing new members join, and seeing mutants join the broader Marvel universe.
Sadly, the warped mind of Grant Morrison thought the x-books needed a shock to the system and created a mutant genocide to sell his new brand of storytelling at Marvel and change the status quo of the X-men. The writers that have followed seem to have tried very hard to imitate that style. Decimation then made things a lot worse and now Dark Reign isn't inspiring a lot of hope right now.
This morbidity is a recent trend, I think, and a company wide directive, all to make the mutant plight even more desperate and urgent. Sadly, the stories thus far have made things worse and have been single minded in their vision. The X-men and the mutant population they are protecting have had to isloate themselves from society, segragating themselves from humanity and they increasingly become more of an army, than the family that made so many fall in love with them in the first place.