Yes the part of the hotel that was bombed was the part of the hotel that was used by the British military thereby invalidating your value judgement that the attack was meant to kill civilians.
The bombing killed many more civilians than they did British: "41 Palestinian Arabs, 15-28 British citizens, 17 Palestinian Jews, 2 Armenians, 1 Russian, 1 Greek and 1 Egyptian" according to WikiPedia. Not a surprising result when setting off a huge bomb in a hotel, even if in a specific area of a hotel. The list of the organization's previous attacks includes many directed against civilians (http://en.wikipedia.org/wiki/List_of_Irgun_attacks). If the killing of civilians was indeed not part of their aim, you would think the organization would treat the operation as a tragic mistake and distance themselves from it rather than coming together to commemorate it in 2006.
We have justification and apologies for violence right here! This is part of the problem, because similar arguments from the other side will probably be grounds for well, you know what.