A major part of Kant's argument for beforehand (a priori) decision making is that (this is Kant's claim) it is impossible to accurately predict outcomes. Even if you can reasonably predict the immediate outcome, there are always unintended consequences, further effects down the line that you cannot predict. Achieving some immediate good might be totally outweighed by causing a greater evil down the road. Kant believed that immoral acts always led to bad results in the end, even when these could not be seen right away. You can think of this as a 'reap what you sow' principle or similar to karma.