I do see what you mean, but he most certainly isn't a Villain Sue, being that he fulfills almost none of the qualities for being one. Broly is quite liked by the community, despite everyone agreeing that his backstory is monumentally stupid. There are many reasons to like a villain like Broly. I mean, he is unique in Dragonball, being that he is pretty much the only pure evil Super Saiyan. He also serves as sort of a dark reflection of Goku, in that he is a Saiyan who's only goal is to grow stronger, but whereas Goku grows stronger to help his friends and family, Broly grows stronger to seek and destroy everything. He is also an aesthetically appealing character, being that his "SWOLE" thing is, again, not something we see in Dragonball Villains. Most villains in Dragonball are of a much sleeker design to emphasise that they are not only immensely strong, but also immensely fast, whereas Broly's design only shows off his immense power. You don't fight him, you throw everything you have at him and he just takes it in his stride, "The Unstoppable Force" if you would. Broly brought something unique to the franchise and that garnered him some love.

The other main thing about Broly is that the first movie he was in was actually immensely well done in as far as the buildup to his reveal. I mean, all we see is a galaxy being destroyed, and all we know is that a mysterious force did it. They spend the entire movie bigging him up, then, when he's revealled, the audiences response is "Yup, i believe that's the motherfucker who killed an entire galaxy!" as we see him effortlessly take apart the main roster. Other villains like to toy with the team, but Broly is 100% focussed on the kill. It is kinda a breath of fresh air in the franchise for them to take on a villain that doesn't even try to outsmart or toy with the heroes, he just destroys them.

The second movie also brought something new to the table with it being, essentially, a slasher flick in the Dragonball universe, with Gohan, Goten, Trunks and Videl having to give Broly the runaround. It is one of the bleakest of Dragonball movies. Every other villain in the franchise is portrayed with a sense of "Yeah, he's strong, but Goku'll have 'im." There is no Goku, there is no Vegeta, there is just Gohan, his GF and 2 children to fight this otherworldly monstrosity. Not even everyone combined is even slightly a match for Broly, and that's before he ascends to his LSS form. When he finally transforms, they actually need to use a Dragonball wish to defeat him. Again, i know his backstory is stupid, but at the same time, you can't deny that he is unique as a Dragonball villain.

That's all well and good, but that backstory just kills it for me. Like I said, he looks intimidating, his attacks are brutal and badass. You are right that he was given a lot of build up. But that backstory just... ugh... all that interest I had in him just instantly deflated. His only saving grace is that he is the one sole villain in Dragon Ball history that actually requires the "everyone dogpile him" strategy for him to be defeated, since that's pretty much how he was defeated in all of his appearances. I'm noticing a distinct lack of mentions of the third Broly movie, however. Not that I blame you, that movie was just... ugh.

But are you sure Goku actually wants to get stronger to protect people? Because starting with the Android Arc he starts making monumentally stupid decisions and from what I remember about the Buu Arc and what I've heard about Super... he never stops making stupid choices. I mean really, Goku, you're just going to risk letting humanity be wiped out by Androids because you want to train and fight them fair and square? You want to give CELL a sensu bean before forcing your pacifist son into a battle he has no actual interest in? You don't just use your Super Saiyan 3 form to instantly fuck Buu up because you're afraid of hurting Vegeta's feelings? Oh sure, you could just kill Frieza before he turns Golden and not even have to worry about him blowing the planet up, but no, let's let Frieza get away with this crap because as long as YOU get a good fight out of it, then nothing else matters, right? ...Oh dear, I'm ranting, aren't I? Sorry about that, Goku's personality and decisions tend to piss me off more often than not in the later series.