Yes, it is important to require a waiting period in all gun policies. This helps deter people from buying a firearm on the spur of the moment and making rash decisions. It also allows government agencies to do necessary background checks to ensure the individual is fit to own a gun.
The waiting period required for purchasing a gun MAY prevent someone from buying a gun in anger and immediately using it. I do believe, however, that if someone is determined to use a gun, they are capable of waiting out the waiting period and using the gun anyway. Even though there are some people who are determined to wait to do harm to others, some might change their mind during the waiting period once they have had an opportunity to calm down.
A waiting period is definitely a good idea, especially for new gun purchases or when someone is buying a gun they don't already own a kind of, for example the first time buying a shot gun. This avoids impulse purchases and ensures there is ample time to do background checks to ensure the person should have a gun.
I feel that gun ownership should be taken very seriously. I believe that it is important that gun owners get background checks along with a waiting period to do psychiatric evaluations before they can own a gun. I also believe that they should be able to pass a test on gun knowledge and safety. It would save lives.
A Mandatory waiting period is too old fashioned. Because most states require a license to hold a gun, and thus gun safety courses, then it's reasonable to assume that the future gun owner is ready to own that gun. It's important to be safe, but please, let's update this old law.