A participle act as an adjective. But adjective mostly come before noun. This sentence only has pronoun. And "arrested" comes after pronoun. If "arrested" does not act as an adjective, What is its function in this case?
There is a small group of verbs that can take an object followed by an adjective: examples include make, get and consider. According to the Cambridge Dictionary, want is one of these words, and it also specifies that the adjective can be a past participle or a present participle:
I want these curtains cleaned - past participle
I don't want him hanging around here - present participle
Looking at your sentence:
I want him arrested
him is the object of the verb, and arrested is a past participle that describes the state you want him to be in.