i believe that the people that killed karl n russou obviously had a motive but which appeals to be the best one.
first of all lets get the obvious one out of the way it could be richard and the others as there wud be no communication to them from ben, thus making it impossible for ben to warn them about russou, karl and alex coming to the temple. the others wud also in my opinion set up a perimeter as they are expecting a "war".
it cud also be a plan by ben. As i think he wud of known that the others wud kill anyone that came near the camp and that the only person they wudnt kill wud be alex as she is bens daughter and that he lied to them so tht he could keep alex at the temple (apprently the safest place on the island) this wud be in my opinion the strongest of theories as it is very ben like to do something like this although the main flaw in this theorie is that he doesnt kill innocents and karl andn russou are innocent so unless its a michael situation n they were accidently shot then it wud not fit into bens plan.
another theorie i thort is that it could be frank and keamy as they left the island on the previous episode. although there has been no proof they know about bens plentiful amount of money but if they do then obviously they wud attempt an abduction. but there is many floors with this theory as how wud they know what is happening at locke's camp, where there headed and whose with them. but it shows that keamy knows how to fire a weapon and is skilled in doing so this strengthens this theory as there is only one gun man (the direction of the shots and the quantity proves hes a good shot) as they take cover behind a tree and he cant shoot them. knowing that it is bens daughter wud stop them shooting as they cud ransom her back to ben for his money.
it could be a embittered mikhail although this is very unlikely as he is confirmed dead
anyways i hope u find my theories interesting plz comment on my thorts and tell me what you think as im interested in what any1 has to say