Alex says to Jack "Eye for an eye..." when talking about what they're going to do to Juliet in exchange for killing Danny.

Matthew 5:38; was this one of the verses that might have been on Eko's stick?

No...

http://www.lostpedia.com/wiki/Jesus_Stick

unless were gonna find out about Eko's stick in future episodes...maybe get a glimpse of its other side...but i doubt it

The phrase "an eye for an eye, a tooth for a tooth",:a quotation from Exodus (http://en.wikipedia.org/wiki/Exodus) 21:23-27, expresses a principle of retributive justice (http://en.wikipedia.org/wiki/Retributive_justice) also known as lex talionis (Latin (http://en.wikipedia.org/wiki/Latin) for "law of retaliation"). The basis of this form of law is the principle of proportionate punishment, often expressed under the motto "Let the punishment fit the crime (http://en.wikipedia.org/wiki/Let_the_punishment_fit_the_crime)", which particularly applies to mirror punishments (http://en.wikipedia.org/wiki/Mirror_punishment) (which may or may not be proportional). At the root of the non-biblical form of this principle is the belief that one of the purposes of the law is to provide equitable retaliation for an offended party. It defined and restricted the extent of retaliation. This early belief is reflected in the Code of Hammurabi (http://en.wikipedia.org/wiki/Code_of_Hammurabi) and in the laws of the Old Testament (http://en.wikipedia.org/wiki/Old_Testament) (e.g., Exodus 21:23-25, Leviticus 24:18-20, Deuteronomy 19:21).
In reference to torts, the Old Testament prescription "an eye for an eye, etc." has often been interpreted, notably in Judaism, to mean equivalent monetary compensation, even to the exclusion of mirror punishment.