So, this is probably spoiler material.
Later they return to rescue him, having an attack of conscience. What they find is the handcuffs still latched, and a severed hand. Graphic, unsettling, and a nice little moment.
Bugged me to death.
Zombies are trying to get to Merle, but the door is chained shut, keeping them off the roof pretty effectively. There's no time crunch to get out of the handcuffs. Why cut off your hand instead of chain on the cuffs?
I mean, Mad Max taught us all you can hack through the handcuff chain (even high tensile steel) in 10 minutes....