I think the only way that stricter gun control laws could have had an effect on this incident is if the law banned gun purchases/ownership for everyone who ever had any kind of treatment for any mental health problem. Such a law would be unfair to people who have been treated and healed of mental health problems, it may in the long run cause more people to hesitate to receive mental health care, particularly if they enjoy the sport of hunting, and such a law would not have prevented the stabbings and the people being run over with a car that were also associated with this incident.
I sympathize with the parents of those slain, but I also recognize that when one's child dies, there is a natural tendency toward anger and irrationality. If I had a son or daughter who died in this incident, I'd probably want to ban knives and cars as well as guns. But we cannot make laws based on the anger and grief of the victims. Laws should be carefully thought out and ALL of the parties who could potentially be affected by the law, both now and in future generations, must be taken into consideration.