So why did the horrible massacre in South Carolina happen?

Because human beings are inherently corrupt and wicked, and only through the blood of Jesus Christ can we be saved. We're no better or worse that that evil gunman, and all are equally deserving of God's wrath.

Thankfully, God has provided an escape from the condemnation.

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John 5:24

Verily, verily, I say unto you, He that heareth My word, and believeth on Him that sent Me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.

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Romans 3:10-18

As it is written, There is none righteous, no, not one: There is none that understandeth, there is none that seeketh after God. They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one.

Their throat is an open sepulchre; with their tongues they have used deceit; the poison of asps is under their lips: Whose mouth is full of cursing and bitterness:

Their feet are swift to shed blood: Destruction and misery are in their ways: And the way of peace have they not known: There is no fear of God before their eyes.