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Between Persia & India, Alexander had become more ruthless, massacring cities even when they didn't resist.
He always executed any who resisted before Porus/Purushottama.
Why was Porus/Purushottama allowed to live?
Defeating Parvateshwar (Porus) had been a very difficult task. Moreover Sage Dandyayan had told him that it is impossible to win over this country. When asked, Maharaj Parvateshwar told him to behave with him as a king. So he left him.