Former Prime Minister Abe made a speech of support near Yamato Saidaiji Station. According to investigators, the suspect fired shots from about 5m away from behind Abe. Video footage shows that the suspect had been in the area for several minutes [you can see him clapping with everyone in the background]. According to a LDP [自由民主党 = Liberal Democratic Party] official, the security arrangements were likely rushed, since the former Prime Minister's arrival in Nara was a somewhat last minute/impromptu affair.
As for the security arrangements, according to the investigator, one
SP from the Metropolitan Police Department [SP = Security Police, who are assigned as bodyguards to domestic and visiting foreign dignitaries] accompanied him under the direction of the Nara Prefectural Police.

According to Kenichi Furuya [see screenshot above], a former Kanagawa Prefectural Police officer who now works as a private bodyguard, He says that the suspect was probably being held back by local police and that the priority of the SPs is first and foremost to protect those they are responsible for (in this case, former Prime Minister Abe).
The main questions that came up in the interview:
Please tell us about some of the problems you noticed this time.
Firstly, there was only one SP assigned from the Metropolitan Police Department, when usually, SPs work as a team, so I think it must have been quite difficult for the SP to work with the Nara Prefectural Police since they had never met previously. SPs are trained to protect the people they are guarding and take great risks to assure the safety and security of those people. However, I felt that since it was a sudden incident, it took time for the assigned SP to fully grasp the situation. [In another news report (that I can hunt down if anyone is interested), someone who was present at the scene was interviewed. They said that they had first mistook the sound of the gun going off as fireworks and just assumed that it was some kind of a prank.]

Typically, for an event involving someone like a former prime minister, a thorough search of the surrounding area would be conducted the day before, and again on the day of the event. The SP would concentrate on guarding the area and would be on the lookout for suspicious persons. However, due to the sudden change in schedule, it may not have been able to prepare in advance. Looking at the site where the speech took place, the roundabout is surrounded by roads and lined with buildings, making it an easy target.
What do you think of the impact of the sudden change in schedule?
I believe that even if there is a sudden change in the schedule, careful consideration is to be given on how to best deploy security measures. Although we [SP officers] have been trained to protect ourselves and our subjects against knives, I think it was difficult to adequately prepare for situations in which we would have to defend against someone using guns.

According to investigators, the suspect Yamakami said of his homemade gun-like device: "Each gun has three tubes and each one of these tubes contains six bullets. I've made several of those guns, and they were completed around spring of this year." From those comments, it's highly likely that this was a premeditated crime.

Yamagami (41 years old), was also previously a member of the Maritime Self-Defense Force [supposedly served for 3 years from 2002 and 2005]. According to the Defense Ministry, they are still in the process of confirming whether he is the same person, but if so, he knew how to handle small arms, trained in live-fire shooting, and could disassemble, maintain, and assemble firearms.
Yamagami committed the crime with a homemade gun. What do you think about this?
Having been a police officer for a long time, I can handle guns, however, I do not have the knowledge required to construct a modified gun. That said, there are easily accessible instruction manuals that allow anyone to make their own modified guns [as evidenced by some of the materials posted in this very thread lol]. The gun used in this case was rather crude. We also know from the crime scene [presumably Yamagami's apartment?] that the suspect also makes explosives. When he made this modified gun, he likely tested it. He knew that the accuracy would be poor and the firing power low, and so he knew that it would be necessary for him to be close to former Prime Minister Abe when he took his shot.
There are election-specific circumstances where you don't want to distance yourself from the voters. What should be the take home lessons?
In this case, the suspect got as close as 5m behind former Prime Minister Abe, and as an SP officer, if I had known in advance, I could have done something about it. We usually expect (and train for) an attack using a knife, but we could not have anticipated an attack with a firearm. It is my opinion, that in the future, we should review our security arrangements and protocols based on cases such as this one.