>>23547462
Yzak had no way of knowing it was filled with civilians. It had departed from an EA military vessel and had no markings or anything that would've identified it as a non-combatant. There's a whole series of protocols about dealing with this:
>shooting at an APC? Okay.
>shooting an an APC clearly marked with a red cross? Not okay.
Therefore, it was a legitimate target and not a war crime.

>>23547554
>when he found out what he did he was horrified and offered to be executed for it
No, he was put on trial for his actions and demoted, just as Dearka was for defection. He never "offered himself".

>It's like asking why Kira and Cagalli didn't have Shinn executed post Destiny for all the Orb people he killed.
Again, military targets, not targeting of the civilian population. It's like asking why militaries bother having POWs, instead of just executing them for killing people of their side.