Could be. Like I posted earlier, I don't think they can get them on murder. Intent is going to be a long shot to prove beyond a reasonable doubt.
Don't need intent if they have another felony.
Don't need intent if their actions are considered a depraved disregard for human life.
I'm not a lawyer so I don't know what other felonies they could be charged with. If they don't meet the definition of a citizens arrest then is that kidnapping? Is kidnapping a felony? Is it felony assault because they're using a deadly weapon?
As best I can tell their citizens arrest argument doesn't hold up by the state law at all. Not by legal text or by court precedent cited in the news.
So felony assault that led to a death? That would be murder.