requires that the perpetrator of a crime suffer or forfeit his life?

Is "that" here constituting a clause? If so, why use "suffer" but not "suffers"?

Context:

We are deeply disposed to perceive people as the authors of their actions, to hold them responsible for the wrongs they do us, and to feel that these debts must be repaid. Often, the only compensation that seems appropriate requires that the perpetrator of a crime suffer or forfeit his life. It remains to be seen how the best system of justice would steward these impulses. Clearly, a full account of the causes of human behavior should
undermine our natural response to injustice, at least to some degree. It seems doubtful,
for instance, that Diamond's father-in-lawwould have suffered the same pangs of
unrequited vengeance if his family had been trampled by an elephant or laid low by
cholera. Similarly, we can expect that his regret would have been significantly eased if he
had learned that his family!ˉs killer had lied a flawlessly moral life until a virus began
ravaging his medial prefrontal cortex.

-Sam Harris