Chess problem - don't understand the solution Black to move:

[fen "4rbk1/pp3pp1/1nq4p/8/1PP5/P3N1P1/Q2BrP1P/3R1RK1 b - - 0 1"]


I've tried to solve this chess problem - black to move, find the best move. I couldn't work it out so I checked the solution, which was R8xe3. I don't understand why this is the best move - could someone explain please?
1...R8xe3 wins a piece (the knight on e3), because either recapture leads to worse things for White, since the d2 bishop is pinned to the white queen, and the f-pawn recapture opens the possibility of mate on g2:

[fen "4rbk1/pp3pp1/1nq4p/8/1PP5/P3N1P1/Q2BrP1P/3R1RK1 b - - 0 1"]

1...R8xe3 2.Bxe3 (2.fxe3 Qg2#) Rxa2