What can private agencies do to prevent such potential pitfalls (as the one mentioned in the article) in the future?
The only way to reduce the likelihood of such events is through screening and training. However, even that cannot completely prevent the possibility of incidents such as the ones described.
To reduce the likelihood of such problems, the firms need to screen their employees more carefully. They need to ensure that they are only hiring people who are stable and not prone to irrational fits of anger. Once they have hired the employees, they need to train them well. They need to be very careful that they prepare employees to be confronted by (and to deal properly with) people who confront them and/or defy them.
By picking employees who are emotionally stable and then training them well, private security firms can reduce the danger of having incidents like this happen.