Everybody complains against callous treatment of the the police.
I believe we do not use against with the word complain because complain in itself means something against. Can you please clarify this doubt. Thank you.
Everybody complains against callous treatment of the the police.
I believe we do not use against with the word complain because complain in itself means something against. Can you please clarify this doubt. Thank you.
Complain against is not ungrammatical, but it is very rare outside of legal contexts, where complain has the sense lay a formal complaint or pleading. It is used only with persons, natural or corporate, as object of the preposition, because only persons can be sued.
So you might say that indignant citizens bringing suit against the police department for some misconduct "complained against the police". But in your example what you probably mean is
Everybody complains about the treatment of [somebody—criminals?] by the police
meaning that the police mistreated the criminals.
And you probably mean that the treatment was harsh or severe rather than callous = "unfeeling, hard-hearted".
The word 'Complain' is always targeted at something or someone. It is also a negative impact verb. There is no need to double negate it with a word like 'against'. It is irrelevant and meaningless to do so. You are 100% correct with your assumption.
The verb 'to complain' doesn't collocate with the preposition against, but the noun 'complaint' does. So you can have/bring/ make a complaint against the police.