"I was a huge fan, and although the last two seasons were bad overall, the finale really killed the whole show for me. Everything the characters did to survive was made completely pointless since most of humanity got absorbed into a weird alien species with a hivemind at the very end. The only people left alive and human were a handful of the main characters and they're just going to die on Earth with no hope for humanity to live on.
It was framed as a hopeful ending, but, if you think about it for more than two seconds, it's terrifying and tragic. It was made even worse by the fact that Bellamy wasn't there with the rest of the main cast since Clarke killed him. Not having him end the series with the other survivors took a lot of joy from an already weak ending. Even bringing Lexa back — a truly desperate last attempt to make something emotionally compelling — felt hollow, empty and, ultimately, pointless.