Yanky R,
“Offering excuses, and sugarcoating the truth in fact is what makes OTD people think its okay being OTD, and discourages their return to sanity….It very well may put the fear of G-d back in them.”
A very large percentage of people who were frum and go OTD actually do not beleive that frumkeit is correct (for either emotional or logical reasons).
This being the case threatning them that they will burn in a gehenom that they dont beleive exsists or telling them that not keeping the Torah is evil when they dont beleive it is, wont be effective for obvious reasons.
Most people will be willing to beleive in somthing even if it dosnt completly make sense to them if they see that it “works” (leads to a happy meaningful life ect.) and will be reluctant or even refuse to beleive somthing that does make sense to them if they think it “dosnt work” (that it leads to a horrible unfair existence etc.). Trying to threaten somone into beleiving somthing will not only make you unconvicing but also can seem to support the latter feeling.