Why the villain has a flawed motivation and course of action is something that feels so self-evident. This approach that's become more and more common in recent years that every character needs to act with complete logic at all times or else it's a plothole makes for such hollow, dull storytelling, people don't act like that. Of course the villain didn't stop their course of action at a reasonable point, he's beyond the point of reason.
The sword's corruption is what muddies the waters here. But leaving that out of the picture, how logical is it for someone who lost a child to go out and kidnap and torture other children(yeah he did it to lure the heroes but that torturing pushes it too much into being bad for the sake of being bad IMO)? Same with Reva joining the inquisitors that hunt Jedi to ultimately kill Vader for killing Jedi and younglings.
The story directly invokes mythology (Thor, Zeus etc.) in its plot and so is also incorporating that sort of mythic structure for it's villain's origin. Here's a guy who lost everything and at his lowest point was sought out by this corrupting influence (again if you got the impression that he just happened to stumble across the sword and his god then maybe reconsider how insightful those sources actually are) and in exchange for revenge has his initially understandable sense of grief twisted beyond recognition and loses all sense of perspective. It's a guy having visions and basically experiencing apotheosis out in the desert. The film is practically hitting you over the head with Mjolnir about the mythic approach. Excalibur is probably the most famous mythic sword right? Yet we don't really have a fixed origin for it. Yet it's still a core part of one of the most influential stories of all time. So what specifically do you really get out of learning the backstory of All-Black? "700,00 years ago a guy called Hurr made this sword. It was then used to kill the god Durr" What you actually need to know about the sword is that it corrupts and the guy who had it before Gorr died, because ultimately the path it offers you is not one you should be taking.
I know he doesn't accidently stumble across the sword but hears whispers guiding him to it. I said it was very convenient he was within walking distance with the god he has grievances with nearby. Zeus and co appearing here does hurt the infinity saga IMO. Where were they and what were they doing? Half of life disappearing should affect them too.
Again, up until he kills the god all makes sense with what been shown of Gorr so far. The big leap comes afterwards making Gorr a bit like a cartoon villain.(It's Waititi so maybe this was by design for this movie).
Excalibur is a creation of very old story telling so it's easier to give it a pass since it's a product of its more superstitious times.
When I hear the fairy tales of my youth I have a lot of questions too but I accept those were old stories that weren't really about world building but mainly teaching a moral lesson. Most modern day story telling for teens and adults don't get that pass from me.
It's a film that explicitly talks about the dangers of shutting yourself off from love because of a heartbreaking loss. The villain is a cautionary tale set up as a contrast to the hero. It's classic storytelling. I cain't believe I'm having to defend a film I didn't even particularly like to someone who hasn't even seen it.
This is where the sword's corruption muddies the water, if Gorr made all these decisions on his own and later saw the errors of his ways would've been a stronger message IMO. Now it was the sword pushing him all along till it was destroyed and we can't be sure of a lot of Gorr's personality.
Do they need to though? A lot of times movies over explain things and it ruins the mystery.
An important plot device like that I do think would benefit from some lore info and it would also help with the world building if done right. It's a bit too easy how these new Marvel movies come up with things like this sword, the Book of Vishanti, Eternity wish granter(things greater than the infinity stones) only to also be rendered unusable in the same movie and can't affect future stories anymore.