You guys phrase it like she was screwing Yuu in the present time. She made a rash decision to kiss him and it's gonna hurt hikari but hating her over that sounds weird to me.
If it's weird to you it's because you don't seem to know how basic decency between friends work.
You can't chalk up lying, withholding and ultimately betraying your friend as just a 'rash' decision.
She knew Hikari liked Yuu, even if you want to argue she was trying to supress her feelings etc and it was spur of the moment the rest of her actions logically can't make sense.
letting Hikari know she used to date Yuu at some point is the minimum she should do as a friend especially since she's framing herself as supporting the relationship as much as she can.
Even if you want to act like it'd be awkward etc and make defence's if a person like that existed in real life I'd hate them based on principle for the same reasons.
The reason Yami is hateable is numerous tbh
Her betrayal is only one aspect tbh and while it's the biggest it's far from the last.
The author trying to make her sympathetic ( and failing miserably) dedicating literally a third of the total released chapters on her, her dumbass reason (or lack thereoff) of breaking up with Yuu and just in general being a bland and straight up boring character who's only appeal so far being "im broken and therefore im deep" just makes her obnoxious.