Sterile neutrinos below electroweak scale can solve baryon asymmetry of the universe, tiny neutrino masses and a candidate of dark matter at the same time. In this work, we derive the kinetic equations for density matrices with exact momentum dependence holding total lepton number conservation. We evaluate the amount of baryon number generation more accurately by these kinetic equations. Furthermore, X-ray observations give us very severe constraints on the magnitude of interaction and the range of mass of decaying strerile neutrino dark matter. However, the enough suppressed Yukawa couplings cannot be realized as long as seesaw is preserved. We call such requirement of fine tuning as “Longevity problem”. This problem can be solved by split flavor mechanism in which we introduced multiple B-L Higgs fields and a flavor symmetry. The predicted X-ray flux can be just below the current experimental bound naturally