Dismounting a knight seems like a really good tactic for disabling them...
Attacking other horse lancers is generally a bad idea, but if you're going to attack them it's to disable them, not to necessarily kill them.
If I dismount you 1 km from the rest of your team, you're useless. You think people should "play nice" so that you can go kill half of their team? So you also think that archers shouldn't shoot you in the back, and if someone has a shield and you don't, they should put it away and duel you?
Give me a break, learn to play. There's no honor in war, the sooner you realize that, the sooner you will use tactics to your advantage. The honorable die young. When it's a fight to the death, fuck honor. Bite, scratch, fight dirty, fight like a girl, do anything you can to survive. Honor gets you (and your buddies) killed.