Asalamu alaykum.
Ha, I know you didn't mention me by name but let me just throw in my 2 cents. You can discard it if you wish.
The issue is, as citizens (I hate that term) of this country, we have a treaty with them as they have given us security so we're not allowed to kill / maim them. There's some other variables and Allah knows best.
You should check out Abu Basir al Tartusi's site. The last I heard of this sheikh, he was fighting in Shaam so Alhamdulillah, he's someone who acts upon his knowledge.
http://www.en.altartosi.com/Covenants.htmQādhī Abu Ya’lā in his book Ahkām al-Sultāniyah P152 states:
If – any Muslim - enters into Dār al-Harb with security (Amān) or he was taken as captive with them and then released and given security, it is not allowed that he kills them or take their wealth, upon him is that he ensures security as they do so to him.
Ibn 'Ābidīn in his Hāshiyah 4/166 states:
If a Muslim enters Dār al-Harb with Amān (security) it is prohibited for him to resist anything from their blood, wealth or honour, as Muslims abide by their agreements. So, if he brings to us something that he has acquired in a way that is forbidden due to an act of betrayal, he is obliged to give it in charity, with this obligation being imposed strictly due to what he has brought out of their lands, and this is because if he has stolen something from them, he must return it to them.
The brothers had Aman so in this case, I believe it was unlawful for them to have killed the guy and Allah knows best.