Theory

[Mortune]

My theory for the 8th gate, it's like all the chakra the body is being burned including the ambient chakra in the bloodstream and heart, turning the surrounding chakra deep crimson. Once all chakra is depleted, the user's heart and blood is worn so thin, they can't survive without a new circulatory system implanted and blood transfusion immediately.

 

[Huang]

Good theory. I like it.

 

[Lyke]

Nuce theory my friend

 

[-immortal-]

The 8 gates Pushes you even beyond your own Limits. So my guess is that your heart is Pumping way harder then its supossed to be and then bOOM, it stops dead , 8 gates been used

 

[AGoodBoy]

i just think they get t0o hot and that's about it.

At 7th gate, guy was so hot that his sweat was evaporating; hence

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scan. Notice how guy states it's not chakra, just all sweat.
Anyway, moving on, the point is, by 8 his core temperature is so high that his blood reaches it's boiling point and basically starts to evaporate out of his body and form the red aura. Bit of evidence to the insane core temperature is his hair and eyebrows combusting.

tl;dr; he gets too hot and evaporates away all his blood, or at least half (about the required amount to be alive) and dies.

 

[The Western Wheel]

Nice theory... but if the muscles contract so tight that bones snap and break during previous gates... I think it's a total breakdown.

Besides, according to your theory, one might say the 7th gate is lethal because of dehydration.