He had already thought about the potential risks involved if they went after each of the people in the group.
He had also thought about the possible methods that could be used to get to those in the group along with there being the best possible location to carry that all out.
Beagle reached the same conclusion that Ash did which was that the field location was the best possible location to carry out any kind of plan that they were to carry out.
Depending on the time of day, it would be quite clear that most people who were out would be avoiding Ian and his friends who were usually loitering around and being a public disturbance.
Not only was it a decent place for use for the plan but it had some hiding spots where there could be more cover so if they happened to go into them or could be lured then they would not be within the view of others.
Most things could be carried out without having any observers so long as it was done quickly and quietly.