Look at how skinny the neck area is on that snake. No way that was an adult rat attack. If I were a betting man I'd say adult mouse. They are much more likely to bite.

I have snakes that eat FT and others that only take live. Yes, it's very important to monitor the animals when offering live, but usually the extreme examples liike the animal pictured in OP is from either a sick (again the very thin neck, dropped sides and visable spin line) or a animal in the sheding process. This also took a LONG TIME and as was been stated, NEVER LEAVE the prey animal in OVERNIGHT