I got into an argument with a friend at a LAN party...

[Doctor Jeff]

If the mobs aggro you, you have a right to defend yourself, but if they are just passing through, then that's kill stealing.

small anecdote:

I was PLing a character in mistmoore on the ledge and because of crappy pathing, my mobs would sometimes path through the CE. (one at a time, the character I was pulling with was level 60)
I spam clicked my flux staff (easily 25-30 times)for insane aggro and then watched the ranger in the CE group cast 5+ snares trying to get aggro before rooting my mob and then doing a /shout similar to "whoever is training CE, I have sent a petition"
Guide showed up, I explained what was going on, the guide disappeared, and I didn't have trouble with the CE trying to root my mobs anymore.
(if you pull from cy to ledge they path through the castle and climb up the wall)

So it really depends on how you got aggro.
Did you guys generate the aggro, or was it proximity?
Did the other party attempt to regain control of their mobs?
Did you give them a chance to try to rectify the situation?