I'm American - I know nothing about soccer.
But in sports, generally, saying that someone is a "killer" or that another team are "killers" means that that player is very aggressive while they're playing *and* they win. You would often modify "killers" by specifying a part of the field where they really play hard ("they're killers in the end zone") or specifying a part of the game ("she was a killer in the fourth quarter") or a specific strategy of play ("they killed us with the triangle offense").
Unfortunately, I don't know what "in the box" is for soccer (again, American :) )- it seems like it could be part of the field or it could be a style of play. Either way, it sounds like that team is very, very, very good at playing soccer.
The reason that people were asking for context is that "killers in the box" could have referred to someone/something literally killing someone else in a box.