-They were arrested violently
-They were violently arrested.
NOT A TEACHER
(1) Some experts feel that there may be a difference.
(a) Your sentence is "They were arrested."
(i) If the word "violently" is absolutely necessary to express the meaning that you
wish to give to the listener/reader, then those experts suggest putting it at the
end. They consider the word to then be a complement. That is, it completes the
meaning of the sentence.
(ii) If how they were arrested is NOT 100% necessary for the listener/reader to know,
then it could be placed before the verb. It would then be an adverbial modifier -- NOT
a complement.