Losing a war doesnt mean that every men will be culled, thats what retreat looks like, you cut your loses and make a run from battlefield.
You have to understand the odds stacked against Porus, who was not more then a landlord and had an hurriedly army mostly of peasants, against the well oiled machine like army of Alexander, super power of that time. Alexender army outnumbered Porus by some margin. Porus was defending his ground, he stood his ground and not let an inch go to the Greeks, which eventually cut their loses and made the retreat down the Indus. Infact Porus added more territory to his lands after the battle, hardly an act of defeated man. The accounts you see from western sources, they are all based on Greek historians who wrote the account of this war after 300 years after the event. Clearly the silence of 300 years says it all. And then this narrative that Alexander give away Porus his land back as mark of respect towards his bravery are utter non sense. Just before facing Porus in Jhelum (Pakistan), Bessus of Bactria (Afghanistan) fought Alexender tooth and nail but when he lost, Alexander cut his nose and ears to dishonour him first and then crucified him ordering his men not to let vultures eat his body. Then ofcourse, the treachery to the people of Swat (Pakistan) who also fought bravely against this foreign invader, after coming to a truce with people of Swat, he butchered them in the night when they were unexpecting. And here we are with Porus army inflecting heaviest casualties and sufferings ever received by Greek Army under Alexander, in all wars he fought, and he will honour him by given him his lands back! You can call it a BS, a figment of imagination by western historians.
Alexander was a general, he didnt fight the wars, rather rode his famous black horse Bucephalus in all his Asian campaign including conquest of Persia, surrounded by his bodygaurds, overseeing the battlefield. Here is the interesting thing, Bucephalus died in battle of Hadaspes. You dont have to be rocket scientist to come to conclusion that Alexander protective circle was uprooted, and when his horse was brought down, you can only imagine the fate of the rider.
Indeed Porus son died, defending his and his father lands. Here is the interesting thing from history. You know Mangla? Mangla Dam, Mangla fort, Mangla garrison ? They are named after Porus daughter, Mangla. Some legacy of a defeated man!!
According to marshal Zhukov, man who oversaw Nazi retreat from Russia, Alexandra fate was actually worse then Napoleon here in these lands of ancient Pakistan.