What is a good emotion/feeling/thing/ideology which becomes dangerous in even a bit more amount?

All things. Hope is a great thing. You don’t even have to modify the quantity of hope all you have to do is add misguided to the equation for it to crumble. Love is a great thing, but to do so blindly without changing the quantity also makes it dangerous. It is less about the excess of quantity, but rather more the way in which it is used which adds the aspect of danger to the situation.