Sirius and Remus do say that he was living as Scabbers to escape the DEs who thought he betrayed Voldemort, so escape was probably very much his plan of action. And he would have been too scared to work out that he could have gotten away with killing Sirius. Also, in killing Sirius he would have been free, and would have had to explain things to Remus, who was probably better placed to pick when he was lying. It may also have been impossible for him to kill Sirius, it may simply have the line he could not cross. Sirius knew what Peter had done, and was finally focusing all his attention on Peter, and had been there right from the beginning. Peter deserved to spend 13 years as a rat, and Sirius did not deserve to die. I'll probably reassess my feelings about Peter after MQF (book seven), but for now I think he had never planned for things to go the way they did. I always think he thought that James (hero as he was) would defeat Voldemort. And it's fear, but also shame and horror that drives him underground. He also may have thought that Sirius would get off and the world would be looking for him.
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Also, in killing Sirius he would have been free, and would have had to explain things to Remus, who was probably better placed to pick when he was lying.
It may also have been impossible for him to kill Sirius, it may simply have the line he could not cross. Sirius knew what Peter had done, and was finally focusing all his attention on Peter, and had been there right from the beginning. Peter deserved to spend 13 years as a rat, and Sirius did not deserve to die.
I'll probably reassess my feelings about Peter after MQF (book seven), but for now I think he had never planned for things to go the way they did. I always think he thought that James (hero as he was) would defeat Voldemort. And it's fear, but also shame and horror that drives him underground. He also may have thought that Sirius would get off and the world would be looking for him.