"Evil" isn't a synonym for "dangerous", the writer made Karin say those specific words instead of using words like "dangerou"s for that reason, to avoid that confusion. As you may forget Karin cares for Sasuke, and is on her way to his aid when an unidentify person is standing over him as he is dying rapidly. She is a sensor type, she can identify any one on the battlefield, like good medics or comrades if they were to come to the rescue but this specific person she can't. SO OF COURSE by human nature she is worried which is why she "has a bad feeling".
EVEN IF we used your logic "it must be someone evil, since when shisui is an option? He is not even evil. He's a GOOD GUY. Introducing someone new as a new evil uchiha makes no sense and doesn't fit in way despite having no evidence supporting this.
1. Look up the definition of foreshadowing. Also why would Kishi add suspense for no apparent reason?
2. Reread my thread. I have said that Shisui is a good guy, however, Uchiha might generally have a dark aura around them due to the nature of their power. See chapter 619 page 14.
This is my point, when you have a character that can be either backed up by evidence either to prove the identify is or isn't that person, THERE IS EVIDENCE GIVEN. There is absolutely NO evidence backing up ANY other characters returning other than Kabuto regardless if you feel that there is evidence disproving it. Infact there is evidence DISPROVING the returning of other characters like shisui and zesty. Your post uses this logic:
Character A: Has evidence disproving it more that proving it debunked
Character B: Has evidence Proving it more than disproving, but since the all the supporting evidence can't be answer at this time debunked
Character C: Has little to no evidence to be proven but slightly more evidence to disprove it By canceling the first two it MUST be C
Conclusion: It is Kabuto. He is less far fetched, more recent, and doesn't add complexity and ambiguity as any other characters choices, and WAY More evidence proving it than disproving it. You asked "why would Kabuto turn good evil?" That is the point of a story in case you've forgot, if EVERYTHING was answered or can be easily guessed before the truth was reviled, the hype would be so exciting now wouldn't it?
OT: People asked "why would obito turn evil so it can't be tobi, because obito was good" but as the story progressed it answered it. Sounds funny right? You are doing the same to Kabuto. People sometimes tend to stray away from the obvious choice and created other "what ifs" in there own made up plot twists hoping it mirrors the imagination of the writers.
Character A: Obviously if something has more evidence disproving it then it is most likely wrong. I cannot prove that the mysterious man is Kabuto, but I can suggest through extrapolation that it is not.
Character B:If something has no supporting evidence then there is nothing holding it up, therefore caution must be used. It does not mean that it is wrong, but rather unlikely because it has no support.
Character C:This is the exact same as Character A just worded differently. However, since you suggest that there is slightly more evidence to disprove it then proving it, it is a more logical choice than Character A. However since Character B has no supporting evidence to suggest it validity then it is nothing but a gut feeling.
For example:
I have a gut feeling that I will win this hand in black jack. There may be no supporting evidence that I will win (assuming that I am not counting cards), but there is a possibility that I will win (evidence because you cannot deny the fact that there is a possibility to win). I cannot say for certain that I will win based off of a possibility alone without knowing the parameters and dimensions involved, therefore I cannot use a gut feeling as a basics for an argument. Thus it is weaker then A because A at least has supporting evidence, so then since C > A, therefore C > A > B.
"if EVERYTHING was answered or can be easily guessed before the truth was reviled, the hype would be so exciting now wouldn't it? "
My thoughts exactly. If can be so easily guessed that Kabuto is the mysterious man, then why go through all the trouble to add suspense and foreshadowing. Wouldn't be very exciting now would it?
You still haven't answered my question of why Kabuto would turn evil instead you pushed under the pretext of plot. Furthermore, the reason as to why Obito turned evil could be derived before he was revealed to be the masked man using the variables: x (kakashi regularly visited a grave) and y (Obito told Kakashi not to let Rin die).
"Evil" isn't a synonym for "dangerous"
You are right, evil is not a synonym for dangerous, but if something is evil you can imply that it is dangerous.
For example:
I sense and evil aura coming from the stone. One can imply that the stone is dangerous since it is giving off an evil aura.
