But you're right, we do think so little of mass shootings that we refuse to regulate the access to firearms. And we are absolutely correct to do so. 100 deaths per year in a country of 300 million is negligable.
Although mass shootings get all the headlines, controlling access to firearms will save a whole lot more than 100 lives per year. Most of the savings will come from reduced accidental deaths and suicides.
There is a widespread belief that having a gun in the house makes you safer: this is not true.
In the 1990s, a team headed by Arthur Kellermann of Emory University looked at all injuries involving guns kept in the home in Memphis, Seattle and Galveston, Tex. They found that these weapons were fired far more often in accidents, criminal assaults, homicides or suicide attempts than in self-defense. For every instance in which a gun in the home was shot in self-defense, there were seven criminal assaults or homicides, four accidental shootings, and 11 attempted or successful suicides. source
(other sources along those lines)
There is also a widespread belief a person who dies from suicide would have done so no matter what method: this also is not true. Most suicide attempts are impulsive acts, and most are unsuccessful. An impulse act with pills or slit wrists is unlikely to succeed: it takes time, the person may have second thoughts, and usually recovers through medical and psychological treatment. A suicide attempt by a gun is much, much more likely to succeed. If that suicidal person did not have ready access to a gun, and had to resort to a different method, the changes are good that most (i.e., more than 50%) of those people would still be with us today.