I thought that was a little strange, but after looking it up, it’s probably a defensible phrasing.

He was only convicted of two murders, while the definition of “serial killer” is three+ murders (plus some other qualifiers). The list of other killings he was suspected of doing, but which were never proven or convicted, would put him in the serial killer category. So it’s fair, in a journalistic sense, to say he was “an accused serial killer.”