Li Yifei had encountered Death Qi before, but it wasn't just one entity; the aura of death is divided into many types. The Death Qi that Aliza and the Fire Sect practiced is a kind of Worm Technique. It's hard to describe; this sect has been passed down for hundreds of years, specialized in death, aiming to master it. As a result, the whole sect ended up in a suicidal state. The techniques cultivated by such a bizarre sect are indeed not easily decipherable.
However, Aliza was unwilling to speak out, and Li Yifei couldn't just kill him outright. He had to come up with a way to revive Uncle Bunong while figuring out what exactly happened and why Bunong suddenly collapsed.
Aliza, though not directly killing people, seemed to have a more malicious nature. Death happens in an instant, but Bunong was tormented by this Worm Technique for many days, which made Aliza even more detestable.
