There must be something wrong with me, but this question seemed like so much fun that I had to bump it to the front of the line. Anyway, in chronological order, the most gruesome deaths (I didn’t quite follow your no-suicides rule, sorry) of Middle Earth:
- Fingon: Towards the end of the…
»When he refused to tell Sauron anything
Except for telling him about the Seven, as said in the Unfinished Tales:
Then Celebrimbor was put to torment, and Sauron learned from him where the Seven were bestowed. This Celebrimbor revealed, because neither the Seven nor the Nine did he value as he valued the Three; the Seven and the Nine were made with Sauron’s aid, whereas the Three were made by Celebrimbor alone, with a different power and purpose.
(Also, as is told in the comments by other users, Finrod and Gorlim come to mind, too.)
The description of Gil-Galad’s death though (◕‿◕✿)