A verb for “to die” in Late Notes on Verb Structure (LVS) from 1969 (PE22/152), clearly based on the root √KWAL having to do with pain and death (PE18/91, 103; Ety/KWAL). As such, I would use this verb for undesirable or painful death, as opposed to fir- “to die (a natural or peaceful death)”.
Conceptual Development: ᴱQ. qal- meant “die” in Early Qenya Word-lists of the 1920s (PE16/134), and the root √KWAL had a long history of connection to death and pain in Tolkien’s writings.
This root was connected to death and pain through Tolkien’s life. Its first appearance was as ᴱ√QALA “die” from the Qenya Lexicon of the 1910s, with derivatives like ᴱQ. qalin “dead, dying” and ᴱQ. qalme “death” (QL/76). The latter appeared as ᴱQ. qalme “agony” in word lists from the 1920s (PE16/144). In The Etymologies of the 1930s Tolkien gave the root as ᴹ√KWAL “die (in pain)”, again with ᴹQ. qalin “dead” and ᴹQ. qalme “agony, death” among other derivatives (Ety/KWAL).
The root appeared in both versions of the Tengwesta Qenderinwa from the 1930s (TQ1) and circa 1950 (TQ2) with the glosses “die” (TQ1: PE18/42, 58, 65; TQ2: PE18/103) and “pain” (TQ2: PE18/91). In TQ2 Tolkien connected it to similar roots √GWAL “suffer torment” and √KWEL “fade, die away, grow faint” (PE18/103). Somewhat interestingly, Tolkien gave almost no Sindarin or Noldorin derivatives of this root, indicating it probably fell out of use, most likely crowded out by derivatives of √PAL.