Complex sentence: After endless taunting from Wilson, Francis was given permission by Killebrew to kill him, but after removing his heart Wilson's healing factor first manifested itself, regenerating the mercenary a new heart but also costing him his sanity in the process.
Split into simpler sentences: After endless taunting from Wilson, Francis was given permission by Killebrew to kill him. After removing his heart Wilson's healing factor first manifested itself, regenerating the mercenary a new heart but also costing him his sanity in the process.