I'm kinda creeped out by how many people want to kill cheating LIs 
I think it's a fairly reasonable reaction to the situation, the want at least perhaps not going through with it. Take real life for example, if you find out the person you love is cheating on you, unless you're an extremely passive individual you're likely to want to hurt them in some way. Most people don't go through with it because they don't want to spend the rest of their lives in jail but many many people think it at first. In game form, one can react accordingly and face no real consequences.
Personally, in game form, I'd more likely kill the companion that my LI cheated with as opposed to the LI, because sadly my Inquisitor will still be in love.
Though I do see that a cheating sub plot isn't the most liked idea here, I think perhaps having the PC and a companion both trying to court the same LI at the same time could play out interesting. Perhaps they can have a friendly duel, where the PC leaves the companion a scar so that they remember that day. 20 years later that companion sets in motion a chain of events that brings the realm to its knees.