The fact that you have been part of a “mainline Protestant church” may be exactly why you have seldom seen any threats. It is the churches that are not mainline, that are nondenominational and specifically invite in the homeless, broken, drug-addicted, poor, foreign-born, and problem-plagued that tend to have troubles with security. For these people are those who most need help, and they are also the ones most likely to attract danger.
Yes, and there's nothing wrong with wanting to help our fellow man. But that's become the main focus of so many churches today; and it's a straw man. God's commission to His church on earth was to go forth and baptizing the world into His family of believers; not being a good Samaritan. Being that good Samaritan is part of it, sure, but it's not to be the focus.
Once we lose sight of God's purpose through us, we begin substituting our will for His. And where does that lead us?