I have a party of 4 noobs, 1 is a pro on other games but new to 5e (like myself - but I haven't played in years), 3 are actually playing their first characters ever. Ages run from 13 to 50 (I love a challenge).
I don't like telling players what they can and cannot do if the rules do not explicitly state it, but I also hate to bash new characters for new player naivety. I also try to keep house rules to a minimum (just to make it easier for them to learn the basic rules). So I kind of "chose" to read the Passive Insight score as a social version of the Passive Perception which can "determine whether anyone in the group notices a hidden threat" (PHB Ch.8).
One of the PCs, a 1st-Level Rogue, was setting a trap for a Slaver Ship Captain & Crew to aid the City Guard. To add to the tension, I had the Captain make a snide reference to the human cargo as 'commodities.' The Player, morally offended, snapped back, "shut up you freaking idiot." Caught off-guard, I asked for his Passive Insight and responded, "With your knowledge of the underworld, you know insulting him like that in front of his men will require a violent show of authority. You can say that, but are you sure you want to?"
A little later the same Player went to strike a final blow to a surrendering NPC while others were striking to incapacitate. I did the same thing, ending with, "Your character has been around enough to know that executing a defenseless man in front of the guard would be murder. They might not notice, or they might let it go, but maybe not. Do you want to kill him or knock him unconscious?"
I'm willing to let the Players do the wrong thing and suffer consequences, I just want them to know the consequences ahead of time. The worst part is that I already know the score - it's on the inside of my screen along with his AC & HP, I'm only asking to make it come from his character's mind instead of my mouth. Of course, I fear that come will across as passive-aggressively robbing them of agency - which is why I've only done that about 3 other times in 13 sessions so far.
My question is, am I actually within the rules by doing this?
Bonus question: Is this a actually a spineless way to force character behavior while pretending like I'm giving them free will?