Most of my posts were answers to Mfauli's arguments, thus they were not explained in detail, since in depth explanation of my own views will only deviate from my criticisms of his views. I do understand why you misunderstood, and it is a good thing that you now ask for clarification before concluding on what I actually meant by what I said.
First, in your scenario about the loaded gun, obviously, the person that told him that the gun was not loaded was accountable for the death, since the person that actually shot it believed that the gun wont fire any bullets and therefore cannot hurt anyone. The problem with your analogy is that it does not apply to Sekai's scenario. It is technically a false analogy since the situation is much too different.
First off, it is quite obvious that if you tell someone that a gun is not loaded when it is, the danger that it will injure or kill someone greatly increases. That is in fact something that the person who lied should know, and therefore, any injury or death caused by his lie is his responsibility. However, part (though only a small part) of the responsibility does lie with the shooter, since he indeed didnt check. There is an obvious direct connection from the lie to the death, as causative factor and effect.
As I have already stated 3 or 4 times, Sekai's case (at least the case being argued by Mfauli that she is the "cause" of everything) is different. She may be held responsible for Itou cheating on Katsura with her as a result from her tempting practice sessions (though Itou is also partially responsible as with the shooter), she does not have anything to do with Itou sleeping with Kiyora, or the rest of the gang, as well as running away from his responsibilities. Sekai probably knew that Itou might go for her instead of Katsura because of her actions. She is then partly responsible for Itou cheating on Katsura in terms of motive. But consider this. Did Sekai even think that her actions could possibly cause Itou to become a sex jerk? Of course not. She simply thought that Itou chose her over Katsura since Itou loves her. So then she is not to blame for the following events in terms of motive.
If for example the shooter did shoot someone, and then suddenly realized that he loves to kill (despite knowing that killing is wrong, like how Itou knew that cheating and lying to women was wrong but kept doing it) and started killing huge numbers of people, those deaths can no longer be blamed to the person who lied, since that is obviously the twisted decision of the shooter himself. This basically means that the person that lied did start it, but he was not the "cause" of the entire killing spree, since the weight of the fault lies mainly on the choices of the shooter. I am not saying that the liar is fault-free (as Sekai is hardly fault-free either), but it is absurd to attribute events that Itou (shooter) initiated and conducted without (liar) Sekai's control to (liar) Sekai.
About the mother part, indeed aside from legal liability, there is also the moral issue. Of course the family CAN be angry at the mother for raising her son badly. But they cant be angry at her for killing their family member, since that was an act committed freely by the son. To clarify, I will ask you this: Who do you think the family members of the murdered person will feel more enmity at, the mother, or the killer son? Anger in this case, is reflective of who the victim's family believes to bear the responsibility of the crime. I argued this way since Mfauli was trying to prove that the main responsibility or fault lies with Sekai (which I believe to be untrue), as seen from his posts.
I hope this long post clarified everything. I actually think its pretty clear if you simply understand my previous posts in the context of the entire argument, and not in bits and pieces.
EDIT: IMO, "values" are too complex and its applications too varied depending on each situation to put into words, much less a few lines in a post. If you have to know my "values", I think the best way is to try to discern them from how I explained my views intead of asking for a text definition of it which will ultimately fail in capturing its essence. I do believe though, that simplifying "values" is never the best way of understanding them.