While it's inconsistent in the first few seasons, it's been since established that dying Goa'uld void a deadly toxin into their hosts. At first I thought it was blood toxicity or naquadah poisoning but after collecting the facts and accounting for retcons it finally made some sense to me.
Goa'uld originally injected naquadah as a recreational drug due to euphoric effects on the symbiote, but eventually incorporated it into their physiology to power handheld devices. Naquadah may poison hosts unless it is neutralized by the symbiote (Stargate RPG). [Presumably they acquire naquadah through their diet as with other biologically necessary elements.]
Symbiotes use protein markers to bind naquadah, which is left behind in hosts after symbiote death (SG-1: 3.04 "Legacy"). [Presumably this neutralizes the heavy metal toxicity.]
Symbiote blood is toxic to humans and jaffa (SG-1: 4.04 "Crossroads"). [This predates the mention of symbiote toxin and was never mentioned again. It was presumably retconned.]
Symbiotes release a deadly toxin upon death. Assuming the symbiote is willing and able, it may staunch the voiding. (SG-1: 5.15 "Summit", 5.16 "Last Stand", 8.10 "Endgame", 8.18 "Threads", 10.19 "Dominion").
Cloned symbiotes, incidentally lacking naquadah, are mentioned as dying harmlessly and being digested by the host immune system. No mention is made of toxicity (SG-1: 6.05 "Nightwalkers"). [Presumably these symbiotes were genetically altered to prevent this.]
Why would symbiotes evolve the toxin in the first place? I would assume it’s for hunting or defense, and like other venomous animals they produce their own antivenin. However, if they had venom glands then a rupture would still release it into the host, and depending on how long the venom remains stable then the host's own immune system would decompose the glands and release the venom.
To prevent accidental death the symbiote would need to be able to neutralize the toxin inside its own body before it is released. This implies the bizarre adaptation of antivenin glands, which I would assume are a defense against cannibalism.