"The police only want to help you."
Translation:Polisen vill bara hjälpa dig.
We often use the present tense to speak about future events, but there needs to be some support for that interpretation in the sentence. In this case, I'd take the sentence Polisen hjälper bara dig to really refer to the present, so it would mean 'The police helps/is helping only you' in English.
'The police will only help you [and not him]' should be translated as Polisen kommer bara att hjälpa dig [och inte honom].