One of the key teachings of Buddhist ethics is around abstaining from destroying life. Avoiding killing living beings is the first of the precepts recommended for lay people on the spiritual path, similar to the teaching in most major religions around not destroying human life (though the Buddha included all sentient beings like animals and insects in this protection).
In the Buddhist teaching, the ethical guidelines are presented as training guidelines on the spiritual path, related to learning about cause and effect or karma, and contributing to the positive development of a person's mind and heart. These teachings reveal that killing is bad not just for the victims of violence, but also for the killer. When you plant a seed from any violent or aggressive action, the result can only come to fruition with unpleasant results in the future. What happens in the mind of the killer at the moment of killing is profoundly damaging, and future results for the killer from the act of killing will be strongly negative as well.
Most people have yet to uproot anger and violence from their mind stream completely and have the urge under certain conditions to lash out against another living being. It is good for each of us to recognize that we feel this sometimes. This urge for violence can arise occasionally (when someone insults you or your family, or when someone cuts you off in traffic) or for some people, quite frequently during periods of time. If an individual has access to guns, it is possible to act on these flickering mind states and kill one or many people quickly when a violent intention arises.
Without guns one can certainly make attempts to harm others, but usually with less fatal results (in 2011, 68 percent of all murders were committed with firearms). Limiting access to weapons protects not just the victims of gun violence, but also protects the potential killers themselves from doing something that they might later regret. It can protect someone from planting grave seeds of unwholesomeness, which can only manifest in an undesirable way in the future, whether or not they are ever caught.
Fear is also connected to aggression, and fear is a primary reason why some people want to buy guns. Fear that they will not be able to protect themselves or their family if someone with a gun attacks them. Fear that the government will grow too powerful if citizens cannot have guns. However, it is important to note that those with a gun in their home are much more likely to have someone killed in their home by gun violence (mostly by accident or use in suicide).
Gun violence has been a part of the history of the United States since the arrival of European settlers and throughout the colonization of the country, which was largely achieved through violence and guns. This frontier mentality has been maintained over several hundred years, despite the conditions of life changing. The United States has the highest per capita firearms ownership in the world (88 firearms owned per 100 people, with 40-45 percent of households having a firearm), which is a sign of this mentality continuing.
It is possible however, that we can evolve as a society in the 21st century, and as individuals, to deal with our fear and aggression in less violent and fatal ways. This fear and aggression can be seen as a sign of our hurt, which in bleak moments we take out on others. While we are still flawed as humans, it is wise to foster choices that cause less harm to ourselves and others by limiting easy access to firearms. This is compassion towards our beautiful but still evolving fellow human beings and ourselves, which honors and recognizes both our sacredness and our hurt.