(Ernest does win his fight, but Su orders Vinny to kill him either due to his unorthodox fighting--i.e. low blows and eye gouging although the fights were not fair to begin with and they were fighting for survival--or his embarrassment of Su's fighter).
Su's decision to have Vinny kill Ernest is based on a combination of factors:  Ernest's unconventional fighting style, which was deemed unfair given the circumstances, and his perceived embarrassment to Su's fighter.