Who is John Knox from The Husband and what is their importance?

John Knox has been monitoring Mitch's home from the second story of his garage. When Mitch discovers Knox, the kidnapper pulls a gun on him and begins to walking him out of the garage. Knox, however, stumbles and falls through the guardrail, killing himself. Mitch is afraid that if the kidnappers find Knox dead they will think Mitch did it and kill Holly, so he wraps Knox in a tarp and puts him in the back of his car.