He was a felon with multiple crimes including sex offenses with minors. They were probably arresting him because he had multiple warrants and he reached for his gun.
That’s not what you see or hear on the video. What you hear is that one officer says the man has a gun. Then you see one of the officers draw his weapon and aim it at the man’s chest and he fires.
Afterwards a witness reported one of the officers took the gun out of the dead man’s pants.
With that much established it will be an interesting investigation.