Related: How do I get my PCs to not be a bunch of murderous cretins?
History
The same group of players and I have played D&D 3.5 and Pathfinder for over one and a half decades combined. We are all personal friends and know each other better than we know ourselves, IRL. Our education levels are very different, our professional lives are very different, and our personalities are all very different, but the one thing that always brings us together is RPGs.
Two of us have played D&D, in particular, ever since AD&D 2nd Edition. We have all explored the World of Darkness, and also play consistently together with MMORPGS (mainly World of Warcraft).
Background
One player pitched the idea of playing a "psychopath" for an upcoming campaign based in Cheliax (Pathfinder). First, I asked that person, in the presence of everyone else during character generation, "What sort of psychopath are you planning on being exactly?"
She responded with, "I think I want to role-play a serial killer."
I asked, "Okay, that could be some very good role-playing. However, don't you think that would shift a lot of focus onto your character just by the nature of your nature?"
She replied with, "Well, I don't know." She looked at everyone else, awaiting input, and added, "Most of my serial murders can be done in backstory and perhaps the campaign can be geared towards my evasion of investigation."
I looked at everyone and said, "You all play psychopaths. All of you. Yes, even you." I directed that last line at the church going wife of my best friend, who, in reality, can't even kill a spider without shrieking for assistance.
Wow, here came the "No I'm Not's" and the "I Have Never's."
I then said, "Look at it this way... All of you roll initiative, attack, and kill, without very little thought and foresight. Most of you think the only way to gain experience is to kill something. If orcs, gargantuan sized spiders, and dragons existed in real life, not only would none of you attack it with a gun, let alone a dagger, but you would have insufferable nightmares for the rest of your life. You do all of this without thinking of any consequences of your actions. You believe everything to be necessary for your existence and personal gain. You see every NPC that you encounter along your way as a plaything or a tool to be used or exploited in your grandeur for power. You don't have any empathy, unless it is 'Wild Empathy' on your character sheet, and even then you roll survival checks for hunting game and getting fish. You swat away kobolds like they are flies, despite having an intelligence score and a language. You kill dragons just because it has treasure. You don't care, really, for anything other than yourselves, and if you have a deity, you walk every line and find every loophole so that you can sin without sinning."
The puzzlement and silence in the room lasted for an awkward two or three minutes.
All of my players immediately flipped into reverse. The next gaming session consisted of diplomacy checks every few minutes, overwhelming caring about NPC's, and donations in the thousands of gold to orphanages and churches.
(The quotes above are not verbatim, as a whole. This conversation did in fact take place, but some colorful metaphors and some other off-color commentary hasn't been included.)
Question
How can I get them to come to a middle ground, rather than extremes? How do I convey this without, again, speaking accusingly?
I mean, they act like Ivar the Boneless one session, and Pope Francis the next session. This has now been going on for a month. They are now so self-conscious of their actions that an entire four hour session of play consists of random daily activities that typically don't even need to be role-played.
(I recognise I have, unfortunately, used accusatory language that has been inadvertently offensive. I really need to know how to convey my point to be less accusatory, yet specific.)
Whenever I mentioned this, I got chastised for having called all of them psychopaths. I want to be able to find a middle-ground somehow, though, between these two extremes.
What do I do?