Please, make available to players an option in-game to report kill stealing.
As for now, the game allows to quick report only bots and inappopriate emblems, but it does not cover a thing that happens specially and heavily in areas known for characters to level faster.
I usually report them as “bots” as I have no other tools to make a complaint (I know it can be misleading, but it is the only way to register a complaint right on time). And as these behaviors are unpredictable, taking pictures/videos of these moments to report manually in forums would be hardly practical (both of them would not show each case from the beginning).
Specially in groups, players kill enemies already engaged/damaged by other players way faster, eventually taking from them dropped items due the higher damage done).
It is not fair to just accept players getting some kind of advatange at the cost of harassing others via stealing mobs. In that case, those that kill only their mobs and respect the other players always get prejudiced, because they will always lose experience/items for either groups or higher-DPS characters.