Why is this move considered bad?
It seems that the AI thinks the move is bad because White can live in the corner.  I don't see a way to do it, though.  I think your move is okay.  It's strange that the bot marks the move as bad, but didn't try to live during the actual game.

Added:

I don't think that works, @jknappen, but it may still explain the computer's advice.  After this sequence, which prevents the ko:

$$Wcm12
$$ ---------------------
$$ | . . . . . . . . . |
$$ | . . . . X O . . . |
$$ | . . . . X O . . . |
$$ | . . 8 X . O . . . |
$$ | . 6 7 . X O . . . |
$$ | . 4 5 X . X O . . |
$$ | . b O X . X O . . |
$$ | . 3 . 1 2 X O . . |
$$ | . . a . . . . . . |
$$ ---------------------


White can force at E6, and then has a nice endgame sequence starting at E9.  So it's better to play E2 originally.