Btw I am a straight male.
I think that he is kinda cool. People are saying that they wanted some kind of beast not a pretty boy, and that's respectable. But we already have plenty of beasts like rengar, alistar, mundo, aatrox (kindof), warwick, etc.
I think what they wanted to do here was to
1) kpop boy band. Obviously. I actually look forward to this unlike everyone else I guess. But this isn't the only thing.
2) add more to the lore of the "ionian criminal underworld". This is hinted at in Jihn's description but we really don't have anything else about it. Now it's finally being developed more, and I'm sure with the new zed comic will see even more spotlight.
3) add a "crime boss" archetype. We have gangplank but sett is a bit different, in that he isn't a fat drunk.
4) appeal to e-girls. Again obviously. But I don't have a problem with this considering the literal half of the roster that was meant to appeal to straight males.
5) the fighting game they're releasing. Sett seems like the perfect candidate to be the star of that game, and they want him in lol first.
And for these 5 things sett would make a very reasonable addition to the game and shouldn't surprise anyone. Personally I think he's pretty cool so I don't have a problem with reasons 1) and 4) being absolute cash grab reasons.