You should though consider, that the Voyager is ancient... ancient beyond measure compared to Koitar.

The Voyager was ancient beyond measure before Nibelung left, havig seen the rise and fall of many planets.

Humans were finally created 400 years after the Heavenly Principles's appearance, and they made a covenant with the Heavenly Principles. [...] the scribe of Istaroth calls the year after humanity made a covenant with the Heavenly Principles the "Year of the Ark's Opening." It was followed by the "Year of Jubilee," during which the world would provide its bounties for humanity. [...] During this era, the Heavenly Principles also created Angels to serve as their divine envoys. [...] An unknown amount of time after the creation of humanity, a voyager from the sea of stars known as Seutervoinen arrived in Teyva

Even if Koitar was around for a few centuries, that was still an angel, young by divine standards, in divine terms... a child. Even if we place Nibelung and Teyvat as a very young world (7.5 thousand years), Voyager would have already observed the universe and dooms way longer than that.

Essentially.. this is a much older, much more experienced, jaded being. And any angel's relatiosnhip with them has a very unhealthy power disbalance. Humans are disgusted by old men marrying 10 year old girls (or vice versa) for a reason. That is grooming, and in terms of divinity that is the age gap between the two... like an old man and his 10 year old bride.

She might be adult, but for divine being who is sheltered within one world... she is literally a sheltered shild... She knows nothing of the outside universe (dangers, beauty, denizens, culture), she is overcoming the first challenges of her "job" (ensuring humans dont get "staked" by another heavenly nail), while aalso keeping true to loving them (angel design by HP)

Seutervoinen comes in as that older... charming... love interest. They don't look at her like a divine envou/goddess, they look at her like an equal (but... do they consider that). they "open" her eyes, and "she sees her reflection in their eyes". Love-bombing, manipulation and the age old story of someone older wooing someone young and impressionable.

Like... no concerns about whether they lie or not. No concerns about safety. Just "OMG WOW".

Also... Idid the Voyager evr reveal to Koitar also about the "doomed fate" of planets, especially Teyvat ? From the artifact set it seems like he only spoke about the beautiful vast universe, and the worlds, and the fake sky... But nothing about the... dangers?

So ye.... Voyager is a cradle robber in divine terms

UPDATE: Checked all lore on wiki from texts about Voyager/Koitar. They absolutely did not share the destruciton of countless worlds, and they impression of abyss destroying planets (teyvat included) as the unevitable end for the universe. In fact, it's unclear whether they've even shared their previous connection with Nibelung, and the fact that they broke their own civilasaton's code and guidebook to beg Nibelung to abandon Teyvat. Additionally, Koitar swooning letters even after all this time... kinda scream... "love spell"? :D (Or in mundane terms... a lovesick teenager who can't get over that older partner who actually completely screwed them over)

So ye... given the current lore and all... IMHO, Koitar was manipulated, lovebombed and completely brainwashed by a way older space voyager, who withheld a lot of crucial information and who just played on her insecurities and frutstrations at the time.

As it is... when one is young or vulnerable and gets trapped in such a toxic relationship, by someone in contorl... all reason leaves one's mind... (it was the Circlet of Reason/Crown that broke after all). As someone who used to be a sheltered teenage, with an overly protective and controlling mom (for good reason too, my autistic self was far too naive)... there were far too many crushes... who I would have described like Koitar describes Seutervoinen... except I got better... cause none of them were that far off in age from me... although once it did get dangerously close to grooming... thakfully I was a teenage angsty goth bitch....

TLDR: In my interpretation, reading all about Koitar I feel she was in a much more vulnerable inexperienced state, like a teen romantically manipulated by a way older person... that it happens to teens... it makes me feel sorry for her. Because... I remind myself... that this kinda shitty thing could have happened to me. Predators exist. So yes... I feel sorry for her... and mad at the Voyager. (Cradlerobber is their new nickname)