Magic is constant. People access magic in different ways, but it isn't "gone" after used. Once the spell ends, it returns to its natural state, be it from fire, shadow, frost, arcane, etc.
There isn't one fireball worth less magic in the world after my spell. My Muffin didn't cause a deficiency.
The only people to ever go insane because of magic were those that had it genecially imprinted into their system (elves). Aside from them, everyone else has been dandy.
You have Illidan, who went insane do to his alienation from the moon wells, and you have Kael'Thas and some other Blood Elves, from the loss of the Sun Well. These people literally developed with magic in their blood. Their bodies need it like any other nutrient. When they actually have the magic though, they are fine and happy. So really,it is the absence of magic that is most dangerous in Azeroth.
And before you mention Kelthuzad or other necromancers, read the lore. Their experiments started when the Lich King urged them on by putting thoughts in their head.
The same way he twisted Arthas into sacrificing his soul to "save" his people.
Can magic destroy when used irresponsibly? Yes. But you need to be INCREDIBLY skilled to gain that level of power, and few ( 1 in 10,000,000 maybe?) reach that level without learning control.
The strongest mage I know of is the Dragon Aspect of Magic Malygos. For thousands of years he used incredibly powerful magic, nonstop, and was never detected. The Night Elves foolishly drew copious amounts of magic from the Well of Eternity which took physical(!) form on Azeroth. Note that the Well of Eternity was part of every dimesnion, all time and outside of both at the same time.
That kind of spell is going to be noticed. That well no longer exists in the same form, so a spell of such magnitude isn't a worry anymore.
Magic is addictive, it doens't necessarily "corrupt." The vast majority of mages have lived long, relatively honest lives. And most that start to dabble in the dark stuff had suspicious characters to start with.
You give a murderer a knife, and he can kill a few people before getting taken out.
Give him access to some nukes, and he can take out many more.
I would have to say that, yeah, he is more powerful in the second example.
[EDIT]
Take WoWWiki with a grain of salt. It is a little... unreliable.
Cenarious did teach the Tauren some druidic teachings, but never druidism per se. Malfurion was his first mortal pupil and first Druid. There were NOT Tauren Druids prior to a a year or two following the Battle of Mount Hyjal.
Cenarious abandoned teaching the Tauren, because he was unable to get through to them (although he had better luck than with the Night Elves.)
However, Cenarius' shared ancestry with the Night Elves led him to remain closer to them, even though the didn't listen. It wasn't until Malfurion saught him out that Cenarius actually taught anything that was even in the realm of "Druidism."
What he told the Night Elves and Tauren of the time were probably basic principles that the Tauren did not have the backgrounf knowledge of magic to understand and the vain Night Elves didn't want to.
Druidism is so, so much more.
And as for the bag of reagants, that is why I mentioned a prepared Druid being extremely powerful.
However, in a situation where nature is essentially dead, those seeds basically become single target shots. Without the capabilities to cause them to multiply and blow them around in a wind, their capabilities are very much lowered.
One seed in a natural setting is devastating. An entire pouch in an unnatural one may take out three men, at best.