My friend is a Shillien knight playing in a low rate server, can it do more killing than the usual tanking?

The sk is an offensive tank, so he's almost the best killer among tanks. The best killer tank (in pvp i mean) is the pala, because of angelic icon.

In mass pvp i doubt the sk will be a killer. Sorry to dissapoint you, but the tank is a tank, even if it's offensive. He has the most dps out of tanks, so theoretically yes, he's a killer, but not in mass pvp.

in mass pvp, your job as a tank is not to kill ennemies but your job is to keep your healer alive by taking aggro on you.
On my server, the bigger ally is playing against us like killing mobs.
They protect their healer/dominator no matter what and tanks try to keep target on them while healer are selecting ennemi's target

yes, assist are made on healer wich just select who to kill, healer's have generally more time to see who kill 1st and what to do.