Here's a quote from "1, 2, 3 John: A handbook on the Greek Text" by Martin Culy. I highly recommend the volume.

Quote
Scholars debate whether this term, and related terms, refers to propitiation or expiation. Propitiation focuses on God's wrath being appeased, while expiation focuses on the wiping away of sin. According to Buchsel (317), Plutarch uses this term to focus on both "cultic propitiation of the gods and expiatory action in general." He goes on the argue... [what follows is a discussion of why Buchsel thinks the term does not imply propitiation.] Rather then deciding between a focus on expiation or propitiation, it is probably better to simply recognize that hilaskomai refers to dealing with the problem of sin, while hilasmos refers to the means by which sins are dealt with, or "the means by which sins are forgiven." (LN 41.12).


(Latin phrase goes here.)