As to why Qin Wentian dared to ignore the consequences and even thought of unleashing the Immortal Vanquishing Swordplay, it was because he had cultivated the Great Nirvana Immortal Art. His other true-self was currently cultivating in the royal tomb of Grand Xia. One of his true-self went to Grand Shang to temper himself while the other was constantly cultivating and in mediation, gaining insights of his Mandates.
Hence, even if one of his true-selves were killed here, he wouldn’t die. But of course, Qin Wentian placed an extremely high regard on his original body. If there were any other alternatives, he didn’t wish for it to be killed at all. However, in the face of being surrounded by so many powers, he had no path left before him. In that case, he could only use his death to protect Little Rascal and Purgatory.
Under such a circumstance, he’d rather choose to die.