When you cheat death so many times it messes with your survival instincts I guess.
Kira rather than taking every fight as a serious situation where any slip up could kill him and he'd better always go all out, probably just figures everything will somehow work out.
He spent most of the fight worrying about the ship and how to best get Cagalli back to Orb, even to the point where he told them to just leave him behind and he'd somehow catch up.
If they were truely desperate and worried about surviving at all costs they wouldn't have spent the whole fight making sure they didn't hit anyone directly, they would have sent out the dozen or so Murasame's to start shooting down Zaft's MS's and used AA's guns to blaze their way through.
Which is exactly what Todoka's old second in command spends the whole episode reccomending but Murrue tells him to bear with it because they just want to escape without bloodshed.