Before I go any further, I want to offer my sympathies and prayers to all those who knew the 13 victims, including Alexis. Losing your loved ones in such a violent, sudden manner is awfully difficult to swallow, no matter what your beliefs about life and death are.
The awareness of how fragile our lives are should prompt more compassion, and more willingness to actively pursue the kinds of changes that might bring about a much more peaceful society. So often, though, these incidents quickly devolve into heated, ugly debates about motives or gun control and in this case, religion.
Christianity, Islam, Buddhism, Judaism, etc is not the issue here. The challenges are the same faced by everyone, for as long as we are in samsara. Only varying degrees. We should be addressing fundamental social causes of mental illness and murder. What we should be most concerned with is not so much about trivial discrepancies in which religion but how we might best go about achieving the individual freedom and social equity we want to see by actually having a go at implementing dharma in our day to day lives. Peace be to all.