Before seeing there was an achievement for not killing the engineers I had been going around shooting them. So I just got the Naughty Naughty achievement instead.
I was playing through co-op with my brother and told him not to kill any of them, and when we got to the hive level he received the "Good Samaritan" achievement, but I did not.
Also, do I need to delete my save to start a new campaign over? Or will selecting replay mission from the beginning count as a new campaign?
Just wanted to know the best way to getting the Good Samaritan achievement and I did not find any other threads in my searches.