Why did they go in armed if they did not expect to shoot someone .
Because you dull cu*t they thought there was someone there who might shoot them. Would you instead have someone go up against an armed offender and offer them a cup of tea? Because if you think that's a way forward you can present yourself at the next armed incident and work your magic, that would save someone useful stopping a round and ensure that they use up their ammunition, win win.
Yeah, they burst into a Public property all hyped up full of adrenaline armed to the teeth, if any member of the Public made a wrong move in panic they would have been shot dead,
oop`s wrong property, the right place is fifteen miles away in the opposite direction,
to late once they have shot an innocent member of the Public, even without shooting a member of the Public those that were in the building and subjected to "shock tactics gun law" by the Police would be traumatised.