Hmm, 'The boys are gratuitously bad to the girl' might be a clear direct translation but a bit of an odd thing to say in English. I think 'bad for nothing' or 'bad for no reason' should also be allowed.
yes i agree.. another oxymoron sentence...bad example of English. if you changed the sentence to; The boys are gratuitously helpful or kind towards the girl; it would be a better example. However.. it's the odd instances that we remember easier.