I've been searching for a definitive description of the special abilities for the various weapons, some have some information on forums, wiki/wikia, or guides, but I've yet to see anything that sounds definitive for most of the weapons. The one I'm looking for in particular right now is the Fate ability on the Guillotine scythes. I've found some hearsay that states it either guarantees a critical hit or it increases the odds of executions occurring, but there doesn't seem to have been much discussion on any of those threads.
Due to lack of information I've tested the scythes for a bit myself in-game.
At the time of testing I had nothing equipped other than the Guillotine. This set my crit rate to 3%, crit damage to 400%, and execution chance to 10%.
For a control I used the Angel of Death DLC scythes as well, these had the same stats mentioned above.
My findings: Critical hits are more frequent than the 3% would imply, but not guaranteed. In fact it seems to be between 1 in 5 and 1 in 10 hits are critical. I did not see any increase in execution chance, in fact they were quite rare with the Guillotine.
With the Angel of Death scythes on the same monsters in the area, I saw a lower rate of critical hits, less than 1 in 20. Executions were also quite rare, but I wasn't sure if there was a noticeable difference between them.