Finally! IMO this episode and Black Museum are the only two good ones in Season 4, which was incredibly disappointing because of how fantastic most of the episodes in the previous seasons were.

What I love most about this episode is how at first you are made to sympathize with our protagonist, since the first crime wasn't her fault and she seemed as much of a victim of circumstance as the bike rider was and also because of how she seemingly redeemed herself afterward through years of hard work and good deeds... but then how that feeling of sympathy is used as a weapon against you as the episode goes on. The pacing of the episode is also fantastic, allowing the tension of the story to slowly build, wave after wave until it become almost overwhelming. First with the old boyfriend resurfacing who is clearly unstable and a mess, then as he threatens to reveal all about the past, then as the investigator draws closer, and again once she is caught, leaving you wondering what our protagonist is going to do with her... until that tension slowly turns into absolute dread for what could possibly be coming next as our protagonist continues to dig herself deeper and ever deeper. Until finally all the tension and dread ends in the final few scenes as you come to realize with absolute horror the irredeemable depths to which our protagonist has now plunged, and all sympathy you may have felt is washed completely away leaving you emotionally drained and completely numb inside. And unlike most other "crime" stories, you can't even celebrate the final scene as the police surround her in the school because of how shell-shocked you are... the only thing you can feel is glad that it's finally over so she can't murder any more innocent people.

p.s. @what your strawpoll link is borked because of the trailing ! instead of )
https://www.strawpoll.me/16720600