Per the story, killed by a sniper. Also, so it is being reported. We should wait for further confirmation. Now, I see the BBC is reporting this too.
Saleh party sources say Yemeni ex-president killed in Sanaa fighting
Reuters | Published — Monday 4 December 2017
SANAA: Former President Ali Abdullah Saleh has been killed in fighting with the Iran-aligned Houthi militia raging in the Yemeni capital Sanaa, Saudi-owned Al Arabiya television said on Monday, quoting sources in Saleh’s party.
Al Arabiya quoted a source in Saleh’s General Peoples Congress as saying he was killed by sniper bullets. A Houthi video distributed on social media showed what appeared to be Saleh’s body, clad in grey clothes and being carried out on a red blanket. The side of his head bore a deep wound.
Read more at: http://www.arabnews.com/node/1203586/middle-east