The often used reasoning religious people use justify the existence of evil in a world overseen by an omnipotent and all good god is that without evil there can be no good.
if thats the argument then the logical extension would be evil doers are enabler of good and therefor good.
to use a real world example. Hitler was good because without his evil doing, there can never be the good of Alliances fighting back and saving Europe.
weird argument it is.
update 090802: poppies below in the comments section refer to a passage (Romans 3:8) in the bible that says
Or can we say-as some people slander us by claiming that we say-“Let’s do evil that good may result”? They deserve to be condemned!
so then why does evil exist in a universe created by an omnipotent and all good god?