Omar Mateen was a Hillary supporter. Mohammend Malik, the man who reported Omar Mateen to the FBI (and the FBI did nothing) recalls some of  his conversations with Mateen:

Omar and I continued to have infrequent conversations over the next few years. I last saw him at a dinner at his father’s house in January. We talked about the presidential election and debated our views of the candidates that were running – he liked Hillary Clinton and I liked Bernie Sanders.