First of all, I'm kinda with you. I've watched Élite since the first day it came out, so this episode was hard to watch for me, especially because Polo was always my favourite character as well. Not only because he was so interesting to watch, but also due to his actor who has always had an amazing perfomance, if not the best. So seeing him die and thus, confirming him leaving the show for good was very though.
But coming to his fate, I would have rather seen him going to jail to be honest. Not because he did not deserve death which he obviously did not and everybody will agree with that I guess. It is just a pretty weak argument to be honest. Many characters are sealed with a tragic fate they did not deserve; the best example would be Marina.
However, it feels like the writes only killed him to have that mystery in season 3. Not saying their thought process is wrong but there are so many other ways to create tension and a "whodunnit" storyline. To be honest, I would be more happy with an attemped murder mystery in which Polo just barely survives and goes to jail in the end to complete his redemption. This way, it would have felt more completed to me. And I'm not saying that just because I want my favourite character to live, haha.
Nonetheless, I am still very happy with Polo's arc this season. Unlike you, I am very okay with him getting hated and bullied this season. Of course, he did not deserve it by any means, especially because he tried very hard to redeem himself and win his friends again as you said — but the hate he gets in season 3 was needed. Do you remember when Guzmán beat Pablo Ruiz, the guy who infected Marina with HIV, nearly to death? How would he react to the person who killed her? And how would Samuel, who has violent tendencies as well, react to Marina's killer? To the first girl he fell in love with and wanted to protect at all cost? It would have felt so out of character if they behaved otherwise.
Lastly, I was also very suprised by the lack of emotions the characters had regarding Polo's death. I am not talking about Guzmán because his last scene with Polo was indeed wonderful, but I would have liked to see the others' reaction as well. I think we saw some scenes of Carla in the flashforwards who were, sadly, cutted out in the last episode. But what is about Ander, his other best friend? Valerio, his ex-boyfriend?
Even though Polo would not want Lu to go to prison for her crime and thus, ruining her entire life like Carla said, I still find it a bit unrealistic how forgiving they were to Lu. I expected Ander or Valerio to at least call her out. And Cayetana? To be honest, I was pretty much sure she would rat them out to the police...
Well, but these are just my two cents. x)