Why would trading this bad bishop be a bad move?

Question

When running a game I played through the analysis engines, I get that my 12th move, 12.Ba3, was an inaccuracy (-0.3 -> -1.3).

Since my center pawns are on dark squares, I assumed my dark bishop was a bad bishop and trading it by black's dark bishop, that can also pin my knight on c3, would be a good idea.

What was wrong with this move? Is a tactical error or a strategic one, and why?

[fen ""]
[Event "Let's Play!"]
[Site "Chess.com"]
[Date "2018.05.15"]
[White "me"]
[Black "other"]
[Result "1-0"]
[ECO "C02"]
[WhiteElo "1472"]
[BlackElo "1492"]
[TimeControl "1/604800"]
[EndDate "2018.06.11"]
[CurrentPosition "r1b1kb1r/3q1pp1/ppn1p2p/3pPn2/P2P4/BPN2N2/5PPP/R2QKB1R b KQkq - 2 12"]

1.d4 e6 2.e4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Nge7 { C02: French Defense: Advance, Paulsen Attack, 5...Nge7 } 6.Bg5 Qd7 { (-0.51 → +0.17) Inaccuracy. A better move was 6... h6. } ( 6...h6 7.Bd2 7...Qb6 8.Qb3 8...Qxb3 9.axb3 9...cxd4 10.cxd4 10...Nf5 11.Ra4 ) 7.b3 h6 8.Bc1 cxd4 9.cxd4 b6 10.Nc3 a6 11.a4 Nf5 12.Ba3 { (-0.11 → -0.95) Inaccuracy. A better move was 12. Be2. } ( 12.Be2 12...Bb4 13.Bb2 13...O-O 14.O-O 14...Bb7 15.Rc1 15...Rfc8 16.Na2 16...Be7 ) 12...Nb4 { (-1.00 → +1.62) Blunder. The best move was 12... Bxa3. } ( 12...Bxa3 13.Rxa3 13...Qe7 14.b4 14...Nh4 15.Nxh4 15...Qxh4 16.a5 16...Qxd4 17.Qxd4 ) 13.g4 Nxd4 14.Nxd4 h5 { (+1.02 → +2.10) Inaccuracy. A better move was 14... Qc7. } ( 14...Qc7 15.Nce2 15...Qxe5 16.Bg2 16...Bc5 17.Qd2 17...Qd6 18.f4 18...a5 19.O-O ) 15.gxh5 { (+2.81 → +0.58) Blunder. The best move was 15. Ncb5. } ( 15.Ncb5 15...Qd8 16.Qd2 16...axb5 17.Bxb4 17...bxa4 18.Rxa4 18...Rxa4 19.bxa4 19...Bd7 ) 15...Bb7 { (+0.72 → +2.09) Inaccuracy. A better move was 15... Qc7. } ( 15...Qc7 16.Nce2 16...Qxe5 17.Bg2 17...a5 18.O-O 18...Ba6 19.Ng3 19...Bxf1 20.Bxf1 ) 16.Na2 Nxa2 17.Rxa2 Bxa3 18.Rxa3 Rc8 19.Ra1 { (+2.29 → +1.44) Inaccuracy. A better move was 19. b4. } ( 19.b4 19...Qe7 20.Qd2 20...Rxh5 21.f4 21...Qh4+ 22.Kd1 22...Kf8 23.Be2 23...Rh8 ) 19...Qe7 { (+1.36 → +2.49) Inaccuracy. A better move was 19... Qc7. } ( 19...Qc7 20.Be2 20...Qxe5 21.O-O 21...Qf4 22.Bg4 22...O-O 23.a5 23...bxa5 24.Rxa5 ) 20.Rg1 { (+2.56 → +1.50) Inaccuracy. A better move was 20. Be2. } ( 20.Be2 20...Qc7 21.Qd2 21...Qxe5 22.O-O 22...Qf6 23.f4 23...O-O 24.Qb4 24...Qd8 )  1-0

Answer 1

Tactics.

Trading dark-squared bishops is useful in the long run, but a4-Ba3 is a bit too slow to exchange the Bf8 that hasn't moved yet; Also you weakened the dark square b4 in the process. Black can take advantage of the misplaced Ra3 to attack the weak pawn on d4.

After 12...Bxa3 13.Rxa3 Qe7! {eyeing both a3 and h4}, any rook retreat or protection would allow 14...Qb4, when you cannot parry the simultaneous attacks on Nc3 and Pd4.

14.b4!? is slightly more stubborn but also lose a pawn to either 14...Nxb4 or 14...Nh4.

The game continuation, 12..Nb4??, on the other hand, deprived Black's Nf5 of any retreat square that wouldn't block the Bb8-Nb4 connection. Your smart retort, 13.g4!, won a piece.

By the way, earlier at move 10 you had a possibility to ensure the bishop trade without any weakening: 10.Ba3!? and trade on f8 once the Ne7 moves.