So to answer your questions:
The DayZ map is something that is available for ALL members of the community as IC information. All DayZRP characters do not have to know that all settlements exist, it is at the user's discretion (keep in mind this would only apply to the first encounter of the character with the settlement). Despite settlement rules being more 'exceptional circumstances', Rolle has stated that these rules still do not override the DayZRP rules. As such whilst technically Joe might be breaking the settlement rules, nobody has KoS rights on him as of yet.
In order to gain KoS rights for somebody breaking your settlement rules the kill still has to be fully justified. In Joe's case, him simply being at the camp and rummaging around their things for the first time would not warrant a killing. If he was told to leave and that these tents belong to the settlement and then continued the kill may be seen as justified. So to answer your question the shooter would be at fault and would likely receive and a KoS ban (as there was no prior roleplay in this example you used).
As well as this, the rule:
5.3 You may not focus on out of character gains to the detriment of RP. "Rule play" is prioritizing rules and OOC information above more appropriate IC actions or behavior. Always prioritize role play over rule play.
Also backs up that the killing would be unjustified. If you are to gain KoS rights, you would need a strong in character reason to justify a kill without initiation. For example a repeat offender of this, somebody harrassing a member of the settlement continuously after many warnings etc.
I hope this has answered your questions and if you have any more concerns or queries feel free to ask away.