actually the 'dont kill the enemy fighting your teammate, bump him' that many people say, is, imo, not the right choice.
The horses aren't very manouverable and inf tend to circle around each other and move a lot while fighting, so if you go for bumping the enemy, you mgiht aswell hit your teammate, and you won't be able to stop since you're already at speed and very close when they circle.
A lance, on the other hand, is easier to control, you can quickly change direction and if your teammate gets between you and the enemy, you can cancel the attack.
Also, a lance can not hit both, where you can easily end up bumping both if they are positioned in the wrong way.
EDIT:
ofc this doesnt matter if thge enemy is outnumbered or it is your top player vs. a peasant and other situations where your team clearly has the upper hand. Dont even think about 'taking' the kill here, just find another place where you can help your team more.
Edit again:
Same goes for all the archers shooting into melee fights.... because hey ? why not, they might just end up with another kill.
as an archer i often shoot into melee fights if i see the outcome of the fight being 50/50 for both player or if i think the enemy has a higher chance of winning than my teammate. Why? because there is a bigger chance of me (if you are a decent archer, that is) hitting the enemy than the teammate, so in the long run, shooting into fights like those helps your team. Ofc it goes wrong very often, but if it goes right just a tiny bit more, it is worth it. If my teammate has the largest chance of winning the fight, i wouldnt dream about shooting into it - why risk getting your teammate kileld when he can easily win?
For me, this game is about having your team win, so if shooting into a melee fight works out more often than not, it will increase my team's chance of winning more often than it decreases the chance, which will make my team win more in the long run - so that is exactly what i will do; shoot into melee.