Foregone Conclusion

In the demon descendants community, red eyes represented that they were ready to risk their life. Normal fights between demon descendants would not warrant the appearance of red eyes. 

Only when the demonic blood is boiling and the emotions have peaked, would the red light that signified a demon descendant flash in their eyes.

At this moment, it meant they were ready to risk their lives.

Amongst the habits of foreign species, revealing their true form also signified that they were ready to risk their lives.

Even a foreign species like Ape God who had been normalized, and was almost indistinguishable from human beings, would reveal his true form in a fit of rage. It symbolized the release of nature, and he would become very dangerous at that point of time.