oh to kill her husband and rightfully take his place...
"You keep what you kill."
Rather than starting with cheating, it really seems to have started (and ended, since it's a oneshot) with pre-meditated murder - how romantic! Trading one host of problems for a whole slew of others... that could actually make a fun crime manga/manhwa, the maid/mistress and widow trying to keep not just the murder under wraps and attention diverted, but also their relationship.
The isekai (?) aspects were kind of weird, honestly; not sure what, if anything, it added to the story. I suppose 'love beyond death/reincarnation'?