If all you care about is killing it, you don't need to eval the output. The output just unsets two environment variables which only matters in the current shell context.

  $ ssh-agent
  SSH_AUTH_SOCK=/var/folders/8p/_pwq997168s7vdwwdg_qr1j40000gn/T//ssh-DE0IoJfU5rrM/agent.15015; export SSH_AUTH_SOCK;
  SSH_AGENT_PID=15016; export SSH_AGENT_PID;
  echo Agent pid 15016;

  $ SSH_AGENT_PID=15016; export SSH_AGENT_PID;
  $ ssh-agent -k
  unset SSH_AUTH_SOCK;
  unset SSH_AGENT_PID;
  echo Agent pid 15016 killed;

That said, it doesn't hurt to eval it, so I overstated my case in my original comment.