Perhaps the first mystery in the world of Cradle to really be dug into, the death of the Sword Sage has remained a stubborn mystery. How could someone with the power of a sage succumb to an ignorant plot hatched by a bunch of Jades? Later in the series Eithan remarks to Yerin and Lindon who were low gold and Iron respectively that they could not hurt him even if they tried stabbing him in his sleep, and Eithan is a pale shadow of Tim in terms of power.
There are suggestions that perhaps the Heaven's Glory School found a particularly deadly poison in the labyrinth, but there are a multitude of reasons why this does not make sense. We have seen in Skysworn that deeper portions of the labyrinth require soulfire to enter. Many portions of the labyrinth are dangerous to enter. This poison would have to be uniquely powerful to the point that it would likely be considered a treasure across the whole world, not something likely to be found in the desperately poor Sacred Valley. Finally, they were not even faintly aware of how powerful Tim was, they would have to be unimaginably lucky to acquire a world class weapon capable of killing a sage but also not require its wielder to have power. There is a lot to be said here, but those are the basics, so now the question is, knowing all this, how did he die?
The prevailing theory at the moment is that was not really murdered, instead he let himself be killed in order to provide the perfect circumstance for his pupils advancement. She was in danger, but she had superior understanding of the sacred arts, more skill, and there was no powerful patron who would seek revenge on Yerin for anything that happened in their conflict. Finally, this would also provide her with a suitably powerful remnant with which to advance to gold. Some variations of this theory also guess that perhaps Tim was ascending to the heavens Abidan style and it just appears like death to people around him.
Another theory on how a couple of jades could have killed an Archlord such as the Sword Sage could be that when Tim had been there, he had actually been a Jade! This may seem far-fetched, but in uncrowned when Orthos first finds Kelsa, He complains about how the valley had weakened his powers as a Truegold all the was back to a Jade, this may have also weakened Tim's powers back to Jade, which would also explain why none of the Jade elder's had sensed him as any higher than a Jade himself. The fact that Tim had been weakened to a mere Jade upon entering Sacred Valley would also explain why he would bother sticking around and even insulting them there if not to let them kill him and allow Yerin to bond with his remnant, allowing her the strongest possible remnant for the Path of the Endless Sword