The British had to stop Bismarck before she got into open waters where she could prey on convoys and easily hide from the British Navy. There were at least 3 possible routes Bismarck could use and the British had limited ships available that could take on Bismarck. Hood was also not alone, the battleship Prince of Wales were with her so the odds on paper looked about even. (Bismarck had a heacy cruiser with her)