Considering that this death has not been ruled a homocide, and step-dad is not currently named as even a person or interest, let alone a suspect in this non-crime, the answer to the question is simple:
He can go anywhere he likes, including out of the country.
I read Joe's theory. With all respect to Joe, I don't think it makes sense. First, this guy's post-crime psychiatric issues would not automatically preclude him being considered competent to stand trial -- that's something for the psychiatric experts and lawyers and the judge to determine. Anyway, even if the guy is a gibbering lunatic (not saying he is) then worst case (from the police perspective, and again assuming he did it which I am by no means convinced of) he could be formally charged and the trial delayed until he becomes competent.
If they had the evidence to charge someone they would do so.
I do not have an opinion on how many people were involved or know something. I don't really have an opinion as to what happened here... it could have been a suicide, accident, murder, anything. If we are gonna talk about statistics, then sure, it was probably a homocide, and if so it was almost certainly committed by someone in the home. Further, statistically, if it was murder the crime took place in the residence. And if so, I would GUESS that whoever might have done it, others know or suspect.
It is interesting to note that the cause of death is undermined in about 3% of all autopsies performed nationwide (note that in the Anthony case no autopsy was performed as there was nothing to autopsy, and yet even so a cause of death was produced). So it is pretty unusual. However, there is an intersting thing....
http://injuryprevention.bmj.com/content/12/suppl_2/ii49.full
"Much of the difficulty in conceptualizing and measuring violent death lies in the implicit nature of intentionality, particularly for suicide. Perhaps stemming from this difficulty, a number of studies have concluded that suicides are underreported because of the misallocation of suicide verdicts to other manners of death, such as the accidental or undetermined classifications..." Note: that was a teaser, read the report.
Here is another link, make of this what you will:
http://www.livescience.com/4445-mystery-deaths-plague-coroners.html