Hydelyn is EEEVVVILLL!!! According to some people who won't look at basic thematic readings.
I'm really not under the impression that Hydelyn is evil. I mean, the story could definitely go in that direction, but right now the evidence is shaky at best and doesn't really make any sense thematically. Like, basically on any of the points. I'm splitting up the points below into ideas based on what I've seen, although I'll be using somewhat comical titles.
THEMES YA DORKS!!!
Okay, the is one is pretty simple. If you look at the thematic ideas mainly present in Shadowbringers (so far the most thematically cohesive expansion) then it's pretty clear that Hydelyn isn't the 'bad guy', she's the 'good guy'. The main themes in Shadowbringers are Legacy, Impermanence, and the need for the old to die and new life to be protected and secured. This is a repeated theme throughout the expansion (I'm planning on doing a BIG write up when I get time). The heroes remember the past to push forward its legacy, but agree to protect new life rather than hamfistedly protect the old. Thancred must let the old (Minfilia) die to let the new (Ryne) grow in its place. Y'Shtola willingly sacrifices her life to try and protect the seeds of the culture she's grown. Urianger confronts the ruins of Voeburt, but holds nothing against the new Fey civilization that lives there. By contrast, the villains attempt to sacrifice or kill new life to unnaturally make the present eternal or bring back the past. Vauthry sacrifices the lives of his subjects to make an eternal empire. Emet-Selch is trying to kill the new (The source/Shards) to resurrect Old Amaouritine. Ran'Jit wants to sacrifice Ryne's autonomy to preserve the idea of her as a perfect daughter. So the protagonists (who the writers expect us to agree with) want to let new life live and move on while the Antagonists attempt to pervert the cycle of life.
Which side of that is Zodark, the guy who wants to eat an entire planet's life to ressurrect souls, on?
Which side of that is Hydelyn, who doesn't want that to happen, on?
BUT ALL GODS MUST DIE!!!
I think that this is a bit of the common 'internet atheism' linking in to be honest. People want this to become a 'kill all the gods' storyline since those are popular. I don't think that the game leans in that particular direction since there are many good religious characters, and characters attempting to destroy religion and faith (most notably the Garlean Empire) are consistently presented as evil for it. Our heroes constantly gain aid from supernatural beings (Hydelyn, Hraesvelgr, Midgardsomr, the Four Lords) and aren't presented as weak for it. The only character who screams about freeing humanity from the supernatural is Varis, who is VERY CLEARLY A VILLAIN. There are plenty of games and stories with this idea as a major theme, but FFXIV ain't one of them.
That being said, with the theme of Legacy being so prevalent in this expansion, of putting your wishes forward and entrusting the next generation with them, I could totally see Hydelyn dying at some point in the future. Maybe she has to die so that we can beat Zodiark, or maybe something else happens. However, it'll likely be played for tragedy and drama rather than celebrated.
BUT TYRANT!!!
There's this very strange idea of Hydelyn as an evil tyrant out in the #TeamZodiark crowd. I uh, never understood this? Like at all. What has she ever done other than oppose Ascian interference? It's not like she rules the World as a God-Queen, most people aren't even aware of her existence. She rallies Warriors of Light, but only really defensively and reactively. There is the somewhat juicy idea that she has tempered our main characters, but there's not anything to imply that we don't have control of our actions. People like the idea of Hydelyn as some evil light tyrant since it's a popular concept in various media, but there's just nothing for it in this game.
BUT LIGHT AND THE FIRST!!!
First off, Hydelyn doesn't appear to be in any way responsible for the imbalance/flood on the First. There's no evidence of it, and the fact that people imply it just because she's the 'Light' entity is a little annoying. If she was they would have implied it somewhere. Hydelyn did the EXACT OPPOSITE and saved the first, even sacrificing her Oracle to do so.
BUT LIES!!!
That was written four years ago. I wouldn't be shocked if they hadn't even decided what the Asians actually were back then. It's not really good evidence of anything, especially when Emet-Selch admitted that she has her own side of the story. Next time we talk to Hydelyn we'll probably hear something more accurate and they'll make a lampshade as to her 'lies' earlier.
BUT MUH SIN MEGAMI TENSEI!!!
This is a bizarre repeated thing I've seen on this forum. Using SMT as some sort of evidence. Now, I will start by saying I love the Shin Megami Tensei series. That being said, it's built on very different thematic grounds than FFXIV. Order Versus Chaos is the direct underpinning of SMT, but it's not for FFXIV. Our characters have been on both sides depending on the scenario, but our heroic sides are consistently heroic and the villainous sides are consistently villainous. There's no reason to expect that FFXIV will go in the 'Neutral is always best' direction when There aren't strict Order and Chaos sides.