I typed
/testfor @a[score_ModifyKills=<1](note that "ModifyKills" is a scoreboard objective)
When I gave myself -1 of ModifyKills, the comparator did not output anything. I tried with
/testfor @a[score_ModifyKills=-1]and it outputted a comparator signal.
http://www.minecraftwiki.net/wiki/Scoreboard#Objectives