Proof
Killing intent, bloodlust, pressure, all of these were the same. But knowing that does not mean one would where it came from. So, where did it stem from? One's soul and one's animosity that builds over time. It gets stronger with every life taken and with every spike in one's emotion, in short, it has roots in one's life experiences.
The culmination of one's lifetime, the essence of one's fighting prowess. Though some had greater killing intent than others, the fact remains the same that, like cultivation, it improves with time, and… Wang Ling had been cultivated for tens of thousands of years.
Unknowingly honing it by killing billions of practitioners who got in his way. He slaughtered town's worth of lives at first, then he moved to city's worth, then countries worth until finally, at some point in his life it was said that he managed to kill a part of the continent.