There's no indication he went in too slow, more likely the other driver was passing on the right, and when he saw the merger he sped up to keep him from coming in.
The right thing to do if you are in the right lane is to slow down a bit to let the incoming traffic merge smoothly.
But you know, the other guy was DRIVING WITH A CROSSBOW IN HIS CAR. That has to tell you something about his state of mind. It's not like he drove home and GOT his cross-bow and brought it back, he already had it IN HIS CAR AND ACCESSIBLE, like he was looking for trouble.
So that's my real question -- why was the guy driving around with a loaded crossbow in the front seat of his car?