Why they have Kamen Rider using a power that was meant for evil?
 

Come on! They're Superhero! Why they have to use evil power? They're Batman of Japan! Batman use his own technology. Not evil weapon

The message Kamen Rider always had is imho that it doesn't matter who you are and where you get this power from, but how you use it and what you want to achieve with it.
A lot of (main) Kamen Riders are on the brink of turning evil or losing control, but they ultimately always decide on using their power to help others.
I think this was also mentioned in the two special episodes of Wizard.

^Smart woman. And she's right. Kamen Rider, the original Kamen Rider, was created from evil. Shocker kidnapped Hongo, brainwashed him, and turned him into a cyborg killing machine they were going to call Kamen Rider. But he was saved before that happened, just not before he was made into Kamen Rider. So with his mind intact he set out to dismantle Shocker.

And that makes for a much more dynamic story. Overcoming the evil that created what you use to save lives. Look at Kouta, our current Kamen Rider. He's overcoming the evil that created the Lockseeds and his Driver from the very first episode. Because he knows that destroying two-thirds of humanity isn't a plan. It's genocide.

Its one of those common themes of Rider that persists throughout the years and I'm fine with it. Symbolism of what good can do with the same power as evil could do.

Second thread involving vin13ish lashing on something Toku related, okay, am I the only one who thinks a mod or admin should issue him a warning :confused:?

I think this topic is far from lashing Toku-related, he's just asking why the Kamen Riders fight for justice when the powers they obtain are from evil sources.

The way Rider powers work and how effective they are is on a season by season basis for me. When it is an average Joe, given the power to destroy the world and they make the choice to champion for good I find it a really great if unoriginal story.

Where I find it difficult in Rider shows, is when a character is meant to be "special" or specifically chosen for some kind of giant organisation or ancient McGuffin and I'm just left wondering why they'd pick someone so ordinary. I mean correct me if I'm wrong, but have we ever had a Rider who actually looked like he could handle himself in a real fight? Or run a marathon? And have we ever had a Rider with a genuine military background or some kind of specialised training that suits them for their role? Other than Oren, I can't think of anyone.

Plus why we should care if they're championing over the desire to just use the powers as they are intended if they're meant to be so special in the first place?

Even more than just Kouta. Haruto's, Shinji's, Kenzaki's, and Ryotaro's powers came from their series' monsters, Gentaro, Eiji, and Shotaro's powers came from the same source as their villains', and there's a lot of Riders who were monsters themselves or created by their villains.
It's a big part of many of their stories.

Yup it all dates back all the way to the original show. The whole thing of using the Villians powers for good is just a staple of Kamen Rider and part of the franchise. Having a Rider that has no connections to the villains in that regard just wouldnt be the same.