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.
qual- vb. "to die", pa.t. quallë (PE16:143)