May I notice that the Anime also got more and more bright as time progressed? People may say that the Anime was all happy but lets just compare Gen 1 Gym Leaders to Gen 3. First impressions only.
Brock vs. Roxanne.
Brock was a rough tough guy when Ash first got against him. Having a somewhat crappy past to boot, Brock wasn't exactly the most super happy trainer ever. Roxanne, on the other hand, being a teacher, was a very kind and friendly trainer. Case closed here.
Misty vs. Brawly.
Here's an exception as Misty was already introduced earlier on. Not much to compare.
Lt. Surge vs. Wattson
Surge was a bastard. Having no problems into sending a lot of pokemon straight into hospital, taunting and bullying younger opponents AND hitting on Misty. He was just a tough bastard - until Ash beat him. Wattson was a joke, a laugh, and a friendly ol' chubby. Case closed.
Sabrina vs. Flannery
Ooooh boy, do I really need to remind you how f*cked up Sabrina was? Turning people (including her own MOTHER) into dolls, keeping them captured in a mini city nearly crushing them...can you say dark? Flannery, on the other hand, was a friendly young inexperienced trainer, who kept smiling. Difference much? (Side note: Flannery was a lot tougher on the outside in the game)
Erika vs. Norman
The thing about Erika is that she's a bitch, prohibiting people from entering her gym. Not much of a dark person per se, but she does have a not so friendly side. Norman, on the other hand...yeah...right.
Koga vs. Winona
Koga is perhaps the least notable gym leader in Kanto, being about the only one who Ash beat fair and square in a normal fight. He's not half as dark as the others despite being a ninja. Winona....yeah. Kinda like with Brawly and Misty, there isn't much to say.
Blaine vs. Tate n Liza
Blaine was a broken man who had lost a lot due to all the tourist. But most importantly; he had no problem into commanding his creature made of pure fire to launch a blast of fire at a cute little mouse to send said mouse STRAIGHT INTO A VOLCANO. You see Tate n Liza doing that? No? Me neither.
Giovanni/Team Rocket vs. Juan
We all know Giovanni, to whom insurance money is more important as human lives. Juan is an artist, a preformer, and as such not a dark character either.
See the huge difference? It started in Johto already, with Pryce being the only bitter/dark person among the other gym leaders. Pokemon got bright.