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Big cats will usually only kill in immediate defense or for food. They won't waste their energy and risk their life escalating an encounter when they have the opportunity to run.
It was probably preying on the dog that it killed, then defended itself from the other dog just long enough that it had a clear moment to make like a tree and bust! Jam! Cheese it! Jet! Bounce! Split! Burn out! Kick Bricks! Run dat... erm... yeh... it ran.