I understand your point but I’d consider that an overreaction.
Maybe the Marcus was unaware of the purpose of the locker, a new player perhaps who doesn’t realise the benefit of each classes’ skill cards. Or maybe he really just was a selfish moron.
However, most people may be unaware the weapons locker was specifically for you. It’s not like your name is labelled on the side of it.
In a public game with minimal communication, most people feel entitled to whatever resources are infront of them, wether they put in the effort to create it or not.
Maybe the Engineer intended it for you in your first match, but unless he used chat to specify it then the Marcus wouldn’t feel like he’s doing much wrong. Also, did you use chat yourself to ask him?
A similar point applies to the second example with Clayton as, despite you making it, maybe he thought it was for his Mulcher/Torque Bow as part of the team effort to win.
The third example is interesting, but has a simple explanation. As the other person was also a Demolitions class, he could’ve assumed the Locker was for him and some kind-hearted soul had placed a Boomshot on it for his convenience.
Sure it could sound like excuses for all 3 people but at the end of the day who knows?
It’s Public Matchmaking. You will never find a more wretched hive of scum and villainy.