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Was the squashed rat still alive? If so, it was probably dragging it out of danger, whether that was too late or not. While there is a spectrum from grief to cannibalism among mammals generally, I think rats have only been observed to cannibalise other rats if in distress, e.g. in a food shortage:
That may not have been the case here. I'd say the most likely scenario is that the rat was distressed the other rat had been hit and was either trying to save it and/or hadn't realised yet that it was dead.
Saw a rat in the road on the commute home yesterday. It was trying to drag something into the verge and as I got closer I could see it was another rat which had been squashed by a car. Not entirely sure why it was dragging it out the road but there was something rather poignant about it.