Who Cannot Testify in Beit Din (Mitzvah 75)

(לא תשא שמע שוא תשת ידך עם רשע להיות עד חמס" (שמות כג:א"

The Torah prohibits individuals from giving testimony if they are a sinner (listed in the Mishna in Sanhedrin 3:3). What are the deeper concerns in allowing someone who is a "בעל עבירה" from giving testimony? Why aren't we only concerned with whether the testimony being given is true?

Delivered at the OU Israel Center, May 22nd, 2019 (17 Iyyar 5779)