It's not excuses, it's reasons. It is a fact that Kira had to take care of several suits and missiles launched at AA whilst Shinn had only battle to think about...

The difference between Kira and Shinn is that Kira represents wisdom while Shinn represents idiotism, anger, hate and has a very violent nature. Shinn thinks he is the only person in the world who has been caused great pain due to war.

With people like Shinn there will always be new wars, with people like Kira, those wars will be fought and ended.

PS... again..... Is there any information to who will weild the Strike Freedom or is it still in the dark due to major spoilers?