I Recently watched a documentary about trench life/management and its briefly shown that a German messenger cycles in full view of a group of British soldiers without any reaction from them.
Was it because he was recognised as a messenger? Or since it was only one (unarmed) man that he wasn't worth killing?
I understand that it could be effective to kill a messenger (especially to disrupt a command) but why weren't more killed?
Without more details it's not really possible to comment on that specific case without getting into pure speculation, but messengers/trench runners were not at all exempt from being targeted in the way that a party of stretcher bearers might be given safe passage to do their work in some circumstances (and this would be heavily dependent on the ferocity of fighting in the local sector). Running messages was a very dangerous job indeed, especially bringing messages back from a raid or attack in progress, or moving along a 'trench' line that was made up of nothing but a series of occupied shell craters.
What you describe could be an element of the 'live and let live' system that could evolve in static areas; basically an unspoken (or sometimes spoken) agreement that 'if you leave our working parties/patrols/rations details alone we'll leave yours alone, and we'll all get through this unpleasantness as best we can'.
You might be interested in this answer I posted a few months back which goes in to some detail about the live and let system in the trenches.
That's really interesting. At first I thought there would be some sort of political reason behind this. But a "live and let live" system makes more sense.
Thank you for this!