After progressing the main questline a bit by talking to Markham, you will get full access to the admin vendors in the admin area. Find Amos Ryan, the guy selling armor in the east side and choose the option to test your weapon on his armor "we want an armor demo" which will kill him. Exit the area and enter again and the Ryan sister will take his place. Do the above again on her. This time security turned hostile for me. So if that happens kill security then exit, re-enter and the final Ryan brother awaits you to kill him via dialogue and the achievement will unlock.