Just a game

Yes; it’s always a good idea to step back from heated discussions and fierce arguments to remember that the subject in question is "just a game".
But there’s another aspect of this sentence which is easily overlooked, or ignored. Because a game you play with other people has another component: Social interaction.
Yikes, "social interaction", that almost sound as if we would deal with real people when we play TFC ...
And that’s exactly what it is: A group of people coming together – most often casually - to play a game. It doesn’t matter if that is a computer game, or a ball game or a game of cards. While playing those people are interacting with each other.
Now the nature of the internet (and online games for this matter) helps a great deal to ignore the fact that your opponents are real people, with the wish to enjoy themselves and to have fun. They are not sitting beside you, they don’t talk to you directly, they cannot make physical contact with you. And they cannot make you accountable for your actions (except maybe the server admin who can kick or ban you from the server).
This is the primary reason why lamers and cheaters dare to behave the way they do: The anonymity and the complete lack of accountability encourages lamers to teamkill, to block doors, to hide with the flag.

But "misbehaving" in a game starts a lot earlier than with the actions of lamers. I am thinking along the lines of deathmatching and - to an extent - sniper wars, of all-D teams and people ignoring their team.
Imagine you are in a park joining a game of soccer with a couple of people. Likely you would not even get the idea to ignore the enemy player going with the ball towards your goal. You would likely be annoyed if noone would guard that goal. You would be irritated to find yourself approaching the enemy goal with the ball alone, because your teammates just wouldn’t care to run with you. In most cases such things would just not happen. And it doesn’t matter much if you play with friends or complete strangers (in fact you would rather fool around with friends than strangers). People would likely play the game properly. Maybe not skilled, likely not taking it too serious, maybe not caring about all the rules, but they would basically play soccer as it’s meant to be played.
Yet the same would be almost a miracle to happen on a TFC pub server. And mainly because the anonymity and the spatial isolation - combined with the "single-event" character of a pub game - encourages people to disregard the aspect of "social interaction". People don’t feel responsible for their actions because they are not accountable for them. They don’t care about the other people because those people cannot enforce the following of the rules of the game or of a behavioural code.
People deny that with the fact alone, that they play with other people, comes the responsibility to play properly, to display an appropriate behaviour and to respect the wish of the other players to enjoy the game just as well as they do.
In real life social pressure would most often prevent such behaviour. On the internet we don’t have the means to apply social pressure (or just to a very limited degree). We can only try to lead by example and hope to influence other people to do the same. The more people are doing this the more there develops something close to social pressure. The more people insist on someone to stop deathmatching, the higher the chance this person will actually stop (or leave the server which also serves the purpose).

So ask yourself more often if you would do the same if you would play on a LAN - where you would be exposed to the criticism or even the punishment of your fellow players. Ask yourself more often how your actions affect the fun and pleasure of the other people on the server. Just keep in mind that the more you care about your fellow players and the game, the more pub games will provide a pleasurable experience.