Insanity.
In his younger days, Wei Wuyin was as cautious as a weasel, maintaining a state of rational calm and level-headedness that was unrivaled. It was how he survived and thrived in an environment like the Scarlet Solaris Sect. With steady steps, he was elevated to higher positions within the sect as a disciple.
This was all empowered by an ideology of certainty over risk. Using this, he took his revenge without losing his life after attaining absolute strength to do so. He wasn't the slightest bit reckless.
Everything changed when he obtained the Bloodline of Sin, marked as an Inheritor of this sinful lineage, and realized the depths and potential of what it meant to be 'Blessed'. While his heart of cultivation had been pushed to the verge of collapse by the Calamities of Hell's looming threat and his impending, inevitable doom, he decided to push forward with notably less caution, taking risk over caution.