His own stepson, even.
In some circles, that's known as a call of nature.
His own stepson, even.
Many, many unknown things.
How the relationship was, whether there were drugs involved, so many possibilities that we do not know.
Yes, all such shooting should be carefully investigated. Forunately, only a small number of homicides involve immediate family:
http://gunwatch.blogspot.com/2013/02/the-misleading-murderer-that-you-know.html