Yes, but with just a S3 one in an army you already horror mark the guy up to 10 times before passing out, and then "casting horrors" means you can have him attacked by let's say 3 lesser horrors at once, maybe not killed but surely further horror marking, then he'll meet the hunter of heroes and bye bye
(also researching with Warlocks isn't cost effective, IMHO, they aren't sacred so higher upkeep; I'd use them to blood hunt instead
)