Based　on　a　bootstrap　instability　method,　we　prove　the　existence　of　unstable　strong　solutions　in　the　sense　of　L-1-norm　to　an　abstract　Rayleigh-Taylor　(RT)　problem　arising　from　stratified　viscous　fluids　in　Lagrangian　coordinates.　In　the　proof　we　develop　a　method　to　modify　the　initial　data　of　the　linearized　abstract　RT　problem　by　exploiting　the　existence　theory　of　a　unique　solution　to　the　stratified　(steady)　Stokes　problem　and　an　iterative　technique,　such　that　the　obtained　modified　initial　data　satisfy　the　necessary　compatibility　conditions　on　boundary　of　the　original　(nonlinear)　abstract　RT　problem.　Applying　an　inverse　transform　of　Lagrangian　coordinates　to　the　obtained　unstable　solutions　and　taking　then　proper　values　of　the　parameters,　we　can　further　obtain　unstable　solutions　of　the　RT　problem　in　viscoelastic,　magnetohydrodynamics　(MHD)　flows　with　zero　resistivity　and　pure　viscous　flows　(with/without　interface　intension)　in　Eulerian　coordinates.