Thank you for sharing this so honestly. It takes humility to bring these intrusive images and the anger they stir into the light, and God sees that honesty and meets it with grace.
What you describe is very common for those who have seen graphic violence, even long ago. The mind can replay what the eyes once took in, and those images can provoke anger, distress and shame. This does not mean you are not a Christian or that God has abandoned you. It means your mind needs healing and renewing, and God is able to do that work.
Scripture teaches that we are cleansed entirely through the blood of Christ, not by our own effort. Your past, including what you watched before you knew Christ, is covered by his sacrifice when you trusted him. God does not hold that against you, and he desires to free you from its lingering effects now. He foreordained this redemption and accomplished it in Christ so that you would not remain in slavery to sin and its images.
God also teaches us about anger. Anger itself was placed in us not so we would harm others or live in violence, but as a kind of sting to make us vehement against the enemy and against our own sin. When anger rises because of these unwanted images, you can turn that energy in the right direction. Instead of turning it outward or inward in a destructive way, use it to firmly reject the images, to judge and turn from sin, and to stand against the devil who wants to keep you disturbed. Love, when it dwells in the soul, is like a harbor that brings calm where anger would otherwise bring confusion and tempest. Ask God to fill you with that love and calm as you walk with him. To remember injuries or to dwell on violent images is to practice the soul in evil, but to bring them to Christ is to find healing.
Here are some steps that many believers have found helpful as God renews the mind.
Confess honestly to God what you saw and how it still affects you, and thank him that in Christ you are forgiven and made new.
Bring every thought captive to Christ. When a gore image comes, do not entertain it or fight it with your own willpower alone. Name it as unwanted, turn to God in prayer immediately, and replace it with what is true and pure and good. Meditate on passages like Philippians 4:8, Psalm 23, Psalm 101:3, and 2 Corinthians 10:5.
Guard what you see and hear going forward. Remove access to graphic content, use filters where possible, and be careful with entertainment, news and social media that might reintroduce it.
Fill your mind daily with Scripture, worship and wholesome things. Memorize a few short verses you can recite when images intrude. Singing or listening to hymns and reading aloud can help redirect your thoughts.
Seek fellowship. Do not carry this alone. Talk with a trusted pastor or mature believer who can pray with you regularly and check in on you. If the images remain very distressing or affect sleep and daily life, consider also speaking with a biblical counselor.
I will pray for you now.
Father in heaven, thank you for this dear one who has come to you honestly. Thank you that through the blood of Jesus there is entire redemption and forgiveness for all his past. We ask you to heal his mind. Wash away these intrusive images of gore, and quiet the anger they bring. Give him your peace that guards heart and mind in Christ Jesus. Teach him to use the strength you gave him to stand against the enemy and to pursue what is holy, and fill him with your love that brings calm like a safe harbor. Renew his mind day by day by your Word and Spirit, and remind him that you are near, that you are patient, and that you are making him new. Help him to walk in the light with others who can support him, and to know your presence whenever these thoughts try to return. In the name of Jesus we ask this, amen.
Keep bringing this to God each time it happens, even many times a day if needed. He hears, he is not weary of your prayers, and he is faithful to continue the good work he has begun in you. If you can, let a pastor know how you are doing so you are not walking alone.