In fact, Yang Ming didn't necessarily need to carry it out in such a manner. Yang Ming right now could easily put Ren Jianren to death with hundreds of ways in the blink of an eye in spite of Ren Jianren's surging vigor. Ren Jianren's techniques were like child's play for Yang Ming. Although some of them could harm people, they were not deadly enough.
However, Yang Ming was different. What Yang Ming learned from Fang Tian could easily send Ren Jianren to the YAMA King [1]. But, Yang Ming couldn't do it in the eyes of the public or it would be murder. He wasn't a bad*ss to that extent. If Yang Ming wanted to win, he couldn't resort to assassination, but rather used an alternative method.
Yang Ming knew clearly that he was unmatched with Ren Jianren in a fair fight. He was lucky to defeat Liu Zhaojun last time. His fighting technique merely belonged to the street fighting level. There were lots of flaws facing those who had gone through systematic training.
[1] King of hell in Chinese culture.
[2] A phrase of a poem by the poet, Li Bai. The original purpose of this phrase in the poem is to describe a scenery of a waterfall. In this case, we think that it meant heat is blowing off from his head.