It's against the server rules to run through. I understand people sometimes spawn more than they can handle. That's OK. I don't have any problem with running away if it's hopeless to stay. The spirit of the rule is to try to kill what you spawn.
It's definitely rude to run in and kill another player's spawns, especially bosses. Normal etiquette is to ask if the player needs help before joining melee. Of course, sometimes people forget to control their henchmen or summons, and that leads to problems.
If I see anyone violating the spirit of game play by running through, spawn killing others, or stealing boss kills, I'll give them a warning. However, it's sometimes best to take care of it on your own by first (politely) telling them to stop and reminding them it's against server rules. If they persist, I hear Nuli (and others) has a few characters more than capable of dealing with transgressors.