Are you backpedaling from your claim that the murderer was done shooting?
Where is your evidence that the murderer only planned to commit one murder because, as you claimed, he had a "beef" with him?
If he was a random mass murderer, he would have opened up as he came in, he would start an argument and then shoot. Elementary, my dear Div.
I'd truly be surprised if you've ever won a game of "Clue".
You claim the murderer was done murdering and you now claim he couldn't have been a mass murderer because he didn't start shooting as soon as he came in.
Since you claim to know so much, why did he commit a murder in the first place? What was this "beef" he had with the man he shot? Did it involve a waitress or a bar patron too?