So, the New United States, located in the DC ruins, has had increasingly strong tensions with the other superpower in the region, the city state of Fredericksburg. They have sent a group of 300 soldiers to Prince William Forest Park, and from what reconnoissance soldiers are saying, they are bringing in cannons, guns, and horses to possible try invading DC. So, they President sends a messenger, called Kody, to try and get the prince of Fredericksburg to come and strike up peace talks. He sails down the Potomac and reaches the forest. He has no visible weapons, and is visibly cheery and happy about the possibility of peace, also, he’s singing songs and skipping about, playing with forest creatures. But, the soldiers are suspicious and he is shot on site. I have a question. What would make the group of soldiers, and the prince, suspicious enough of the messenger to kill him?
Other than it being an accidental weapon discharge or one soldier who, psychologically speaking, should never have been given a gun, the easiest way for dear Kody to be shot would be making him look more like a threat.
For example, Kody was unaware he walked through the territory of leper/smallpox colony that Fredericksburg was keeping in quarantine. The guards that saw Kody had orders to shoot any person coming from that direction regardless of excuses, and one greenhorn solider took that order too seriously.
Or, maybe before Kody left DC, Fredricksburg got bombarded with rumors of suicide bombers from the N.U.S. And really, what kind of person really acts that cheerfully when crossing a de-militarized border. Might suspicious if you ask me, my esteemed prince.
Maybe Kody arrived in civilian clothing as a way of leveling the playing field, but put his shiny cigarette case in his vest pocket. He feels like a smoke, moves his hand too fast, a Fredricksburg solider sees a flash of metal, and Kody gets some new holes in'um.
Or maybe the folks in DC underestimated how intolerant soldiers of Fredricksburg could be when they sent Kody, part of a visible minority group, on this assignment. This one is the weakest, as it makes Fredricksburg look cartoonishly evil, but I thought it might be worth including.
There really isn't enough of a reason to ever shoot a messenger unless your military is ready to take force and is highly aggressive. The nature of a messenger is to deliver news, peace/surrender conditions, diplomacy. In that sense, killing a messenger sends a clear message depending on the case.
If its news, it means your ruler/leader has a bad temper. This leads to messengers with bad news of news that they know your ruler will dislike, not stepping forward and being messengers or failing to deliver the messenger. There are a couple examples on the wikipedia page for "Shooting the messenger", but only receiving good news is a sure fire way to overlook enemy activities.
If its related to peace/surrender or diplomacy, then you are essentially telling the enemy or opponent to screw off and that your ready to fight or ignore them and keep doing what you've already been doing.
Onto the actual question part
The most likely way to kill the messenger is that they weren't following the instructions of the soldiers. Likely when the messenger approached the soldiers would ask him to stop and surrender. Disobeying this would lead them to shoot him because he could be a threat and isn't complying.
Next would be if the messenger makes a sudden move or action which could be mistaken for an attack or an attempt to reach for a weapon. Your soldiers and people will put their own safety above that of a messenger, especially an edgy one and hence they kill him to ensure they live another day.
With the way your messenger is acting, they could also think he was diseased, sick or mentally incapable. Basically not a actual diplomat or messenger and hence kill him. Messengers often make it very clear they are a messenger so that they won't get shot, because information can be very important and surrendering early can save a lot of lives or provide more favorable conditions that out right losing and having no choice.
They could also think he is spying on them. Although there would be better ways to restrain or restrict him from being able to spy on them, they could just dispose on him. This ties in with the above, because if a messenger was spying, you could just blindfold them and disorient them before moving them to a secure location.
The final reason would be because you have a goal you are going to achieve, like war, hence you don't need a messenger from the enemy because you are going to fight them.
As an extra note, if it was accidental, you would likely send your own messenger with the original messengers personal equipment as well as some form of compensation and a well thought out apology and explanation of why they were killed. Followed by some form of diplomatic conversation about what you were doing.