Plain old mob mentality, something bad happened, no one likes it, the suspect is dead, someone must be punished, anyone tangentially related is fair game.
Might as well string up the person who sold him a pack of gum last week.
This type of thing is precisely why, after seeing what happened to Richard Jewell after the '96 Olympics bombing, when a tragedy like this happens, I try to stay clear of anything that mentions the killer's name/motivations/etc for a couple days until there is a clearer picture.
24 hour news cycle. Gotta fill the time, and any shred of anything information-like helps add to the discussion.
Every time something happens, the networks fall over themselves to get out the new thing first, and inevitably fuck it up. Recall the Obamacare ruling, and how several networks reported it'd been overturned because they couldn't take the ten minutes to read more carefully.
Putting the information out there isn't the problem. It's reporting it as fact that causes problems. I was watching the BBC coverage of the shooting and pretty much everything new they reported was prefaced that it was speculation, picked up from a US network, or leaked to another network by an unknown source. They were able to fill without stating untruths as fact.
Of course they have logical reasons for acting unethically, just as a common thief has a logical motive for snatching purses. That doesn't make the behavior any less unethical.
Might as well string up the person who sold him a pack of gum last week.