To earn the "RUTHLESS" trophy in Detroit: Become Human, you must kill the Traci androids at the end of the chapter "THE EDEN CLUB".
Enter the club and investigate the two bodies. When you eventually interrogate the android on the floor you'll learn of a second android in the room. Outside, interact with the android opposite the door and then speak with Hank. Get him to pay for the android. Scan it to start a bloodhound trail; you observe where the android appears to have gone and then scan an android in its field of view. On Casual you have a decent time frame to achieve this so just keep scanning androids in the path of the escaping android and eventually you'll be led into a staff area. Locate the blue blood on the floor near the pallets on the left and follow it to the back. Look at the line of androids at the back wall to start a quick-time event fight sequence.
Whether you win or lose this fight you should get an opportunity to spare or kill the Tracis. Kill them to unlock the trophy when you reach the flowchart screen... just be aware that making this decision will make the following trophy a little harder: