You don't want ghosts leading you because they don't learn. It's been hammered in that servants are karmically bound to their fate. They're more like walking and talking history books.
No one ever mentioned "leading" but you bud, mostly because you can't engage actual argument so you choose to shift goals instead. Needless to say, ability to learn has nothing to do with competence of making a choice at given time, and as such it doesn't make non-ghosts any more qualified than ghost when it comes to making educated choices.
All you need to know why Amakusa is a villain is that he wants to remove the "evils of man".
Which is nothing but in built strawman so we can't take his logic as valid. In particular, it shifts argument from "immortality is bad" to "this particular villain is written in such way that his goal is malicious". Basically, Nasu wanted to present his views as something worthy fighting against, so we cheered when Sieg stopped him.
Except "death and suffering is bad" is pretty common sentiment to grand majority of people, wanting to retroactively save everyone is admirable and adding vague line that can be taken as whatever you want only exists as form of saving throw when Higahshied realized that no one buys Sieg's bullshit.
Reminds me of Red Son, heh.
"And so Superman created perfect utopia"
"Uh, shouldn't communism be bad?"
"Fuck, um, make him listen to Braniac's evil plans for no reason whatsoever"
"Genius!"
Also, nice strawman but to act on your example: a kid dying of disease would motivate someone to do something about it. Might not help the kid but itlli help future kids. It's not like the peaceful 1st world countries didn't go through that exact process.
I'm not sure what kind of point are you making here, really. I mean, I think I do, but it's so endlessly fucking dumb that I will give you benefit of doubt and ask for clarification.
Like, surely you don't think that people suffering just to "motivate" other people is something to strive for, in case where such suffering can be avoided, absolutely and utterly? I'm sure my boy Issac made this point quite well some time ago.