I am someone who IS watching the show and am up on all the eps out and while he's definitely better than her terrible adoptive father and better than her first love is gonna end up being, in no way is he a good husband material/choice/green flag/not problematic on anything but a curve compared to the other men:
forced her into marriage as part of his political plans, with no opinion or interest as to whether she's OK with it. Just because he may have gone in with 'ah, I will give her a divorce later' does not negate it in any way. Even if we go with 'oh this is not really historical' doesn't help - if this is a proper period setting, he's fucking with the woman's most important choice/action in life but if it's ahistorical modern dress up and marriage matters less, then all the murder/choking/death threats/etc is not excusable because in a modern person playing dress up they are definitely not good.
repeatedly physically threatens her. He may mean some of it, he may not, fact remains, she's a woman half his size and with no battle skills.
is rude and insulting to her and just acts awfully to her whenever he thinks she's meddling into his family affairs (she is his wife, mind you.)
All the threats culminate in him freaking out she'd reveal his temporary blindness, choking her - literally grabbing her by the neck - threatening to kill her and to destroy her reputation.
his other wife justifies all of this by saying hey at least he's not worse and it doesn't matter if he has a temper as long as it's with his loved ones (which reminds me of the old Russian folk saying "if he beats you means he loves you.")
Now, do I care he's red flag/toxic/ other term du jour? Heck no. My favorite character last year was possibly Di Lin from Eternal Brotherhood 2, who massacred whole villages of civilians on his march to rescue his brothers so the news wouldn't leak. I don't even care if the characters are toxic vis-a-vis a partner. I love me toxic fictional dudes - I am salivating for Overdo to drop and both ML and FL in this one make this dude seem a saint (and the same goes for every FWSC adaptation ML and some FLs) and I enjoy my minis where half of them appear either ML is torturing FL or FL is torturing ML. One of my top 5 cdramas this year was Generation to Generation and ML is a certified headcase; we are not even going to get into what kind of games the bulk of characters of Veil of Shadows, my favorite drama this year and probably in my top 5 of all time, got into.
But! He really is a toxic dude and by any sane, non drama standard, not a great partner. And that's fine, that can make for a story with teeth (my issues with Key are many but ML being toxic ain't one of them) especially if the drama does not try to claim he's green like I am color blind (which this one is rather guilty of) but one can't possibly say he's an ideal dream boat and a great partner! The moment there is choking and a death threat, the man is automatically excluded from sweetheart safe choice label.