"I assumed meant him being arrested." This is not quite a complete sentence. But I don't think that's what's causing your problem. Let's look at the pieces.
The subject and verb are easy enough. The subject is "I". The verb is "assumed". So the sentence is saying that the speaker assumed something.
You have to "assume" some claimed fact or event. Like, "I assumed Bob was tall" or "I assumed the theory of relativity was proven."
The tricky part of this sentence is "him being arrested". This is a phrase acting as a noun. It is basically taking an action -- someone is arrested -- and turning it into a noun. Like we will often say things like, "I saw Sally studying in the library". We are using "studying in the library" as a noun: that is the thing I saw.
In this case, the speaker is saying that something meant that this person was arrested.
So that's the flaw to the sentence: It does say what it was that meant this. Presumably the speaker means that this person said he was trying to prevent a mistake. That is, they thought that what he MEANT was that HIM BEING ARRESTED would be a mistake.
This problem could easily be fixed by adding "it", i.e. "I assumed it meant him being arrested", with "it" referring back to what "he" said.