Boot loop

I bought a used one off eBay. Booted up fine. Side loaded king root. It failed to root. After settings screen was blank flickering. So I rebooted. Same flickering. So booted into recovery and did factory reset. Now it's in boot loop. It gets to upgrading then back to fire logo. Then starting applications then back to logo. Then stays on logo. Been 30 minutes. Tried twice. Any help.