I thought 300 was cool as hell when I watched it, too. There’s just a lot of apocryphal subtext within the movie that lends itself very well to fascist imagery and ideology, like the portrayal of Spartans tossing inbred and deformed infants to the rocks and it being treated as more like an interesting and metal quirk than something horrific. A lot of it is pretty ahistorical, too.

I don’t know if you’re interested in longer form video content, but Big Joel (a supposed lefttuber or breadtuber or whatever, his media criticism is really good imo) has a vid on it where he explains that he really enjoys the movie but has some issues with it – also engaging with fans of the movie who got pissed at him online. https://m.youtube.com/watch?v=hEMqTPUWXqw

edit: I’m not sure you’d agree with all of his points and that’s perfectly ok. There’s probably a coherent argument to be made regarding the movie in favor of it, it’s just that the majority of the people making those arguments online are loud obnoxious manbabies.