Season 1:
- [Mary Winchester]
- Jessica Moore (kinda?)
- [The Impala is totaled]
- John Winchester
- Ash
- The Roadhouse gets burned down
- [Sam dies and is brought back to life]
- [The Colt is stolen]
- [Dean dies]
- Ruby (considered an ally for s3 and most of s4)
- Ellen Harvelle
- Jo Harvelle
- Anna (ally in s4)
- [Sam jumps into Cage]
- Samuel Campbell (ally for first half of s6)
- Gwen Campbell (ally for first half of s6)
- Rufus Turner
- Balthazar (sort of an ally at the end of the season)
- [Cas "killed" by Leviathan]
- Bobby's house gets blown up
- Bobby Singer
- Frank Devereux
- [Dean and Cas get pulled into Purgatory]
- Benny Lafitte
- Meg (sort of an ally s7-8)
- Kevin
- Tessa (not exactly an ally, but worked with them in past seasons)
- Gadreel (sort of an ally at the end of the season)
- [Dean is killed by Metatron]
- Charlie Bradbury
- Rudy (off-screen until 10.23)
- Death (not exactly an ally, but worked with them in past seasons)
- Hannah (not exactly an ally, but worked with Cas in s10)
- Metatron (sort of an ally at the end of s11)
- Billie (not exactly an ally, but worked with them in s11)
- Mick Davies
- Eileen Leahy
- Arthur Ketch (considered an ally for most of the season)
- The Colt*
- Rowena*
- Crowley
- Cas
- Mary Winchester*
Conclusion: Yes, s12 really was a bit of a blood-bath, by SPN's standards. Also some of these characters are borderline allies so feel free to debate their inclusion, and I know I must be forgetting someone--tell me who, please!