You say Mireles was in the fight from the start. If that's true then why wasn't he wearing his body armor? Are you saying he was ready for a gun battle but decided not to wear his vest? Clearly that would be ridiculous. He performed well under pressure but he obviously went into it like the others--not really expecting anything out of the ordinary.
Apparently your definition of being in the fight "from the start" means the start of the work day. Mine is when the fight started, without respect to what equipment they should have been wearing when it did, or their mindset before the fight.
Where Mereles is concerned, his mindset was extraordinary, and he killed both Platt and Mattix, if I recall.
As for the others, I suspect their mindsets before and during the fight were very similar to the majority of LE officers who go to work on any given day.