Antimatterenergy description of Ranma’ arrogance is the perfect argument against an evil Ranma being Vegeta
http://ranmarelated.blogspot.com/2009/10/bragging.htmlRanma’s arrogance is mainly a combination of his casual use of super human abilities, his belief that he can win and refusal to give up, and use of trash talk. But thinking that he would actually stand there and let someone bring an unknown to a fight is to far fetched. If anything it goes completely against his character. His fighting style revolves around making new tactics, copying techniques, developing counters, and discovering a person's weakness. He’s even flat out stated that using a technique against him multiple times is pointless. He’s also incredibly manipulative so someone introducing a new unknown element to a fight that he is already winning is not something he would do. He is arrogant enough to fight a person on their terms as seen with the numerous martial arts and crafts matches as well as to rush in when the opponent already has an unknown element as seen when he rushed Herb fro throwing Akane or when Ryoga first learned the Shi Shi Hokodan
Ranma is also secretive and suspicious of people's actions so intentionally letting his opponent get the upper hand is highly unlikely. Keep in mind that yes Ranma enjoys fighting but the enjoyment is from pitting his skill against someone else. Vegeta but purposefully allowing someone he is fighting to get stronger is to make the fight more entertaining. It is also a product of his Saiyan brawler mentality as Goku, Gohon and Gotenks have all done the same thing. Though that admittedly was by down playing their strength. Ranma does have the ego to do something like that, under estimating his opponent and not going full out, but handing the victory over to them is not something he would do.
I think an evil Ranma would be more like Light Yagami. Read Age of War Horse and I’ll Met by Starlight for the perfect reasons why. In the first one the knowledge of every martial arts style and technique is forcibly inserted into Ranma’s mind breaking it and making him see himself as nothing more than a weapon. I wont go into spoilers but Ranma meticulously plans every detail even his loses are to further advance his goals its only after he knows victory is at hand that he stops looking at all those small details allowing the heroes to get the upper hand. And even then its more a case of he’s focused more on destroying his opponent then just beating them causes his ultimate demise. That’s why I compare evil Ranma more to light then Vegeta deceit and manipulation pretty much define all his relationships he repeatedly takes stock of his circumstances and options, and planning his next move. However he has overwhelming pride in himself and once that goal is achieved he gets tunnel vision and starts making mistakes.
The ranma from IMBS is similar even though he isn’t so overwhelmingly powerful. He’s basically what you would expect Ranma to be like when you hear about his life. He’s crazy and its obvious crazy he’s still incredibly manipulative yet like with the above his ultimate demise comes from over looking a detail.
As for Ryoga I think an evil Ryoga would be Happosai.