When Inuyasha used a Shikon Jewel Shard he became a full demon and was dangerous to all of those who were close to him (probably because of the hatred and anger of his corrupted shard) but Kagome stopped him.
Also Inuyasha has become a full demon in the past even without the help of a Shikon Jewel Shard.
My question is: If Koga (or anyone else who is a full demon) can control his emotions and his temper, even when regularly using the Shikon Jewel Shards, why can't Inuyasha do the same?
A major difference between Inuyasha and several others when it comes to the jewel shard's effects is that Inuyasha is heavily influenced by his human side, having been raised by his mother until her death and without a full blooded relative around to teach him control of his demon side. I've noticed that half demons in the show can lean towards either being more like their human side or demon side. Jinenji and Inuyasha would lead towards their human sides with their love for their human mothers whom raised them.
Naraku on the other hand chose his demon side through and through which is only displayed so surely whenever we see him try to suppress or throw away Onigumo's human feelings for Kikyo. As such, his becoming full demon in power has little effect on his mind because he's already using that side's state of mind to begin with.
Because Inuyasha's demon blood was never trained properly and said blood is so potent to begin with, his more baser instincts are what come out when it over powers his human side. (This potency and his expected death is actually why Inuyasha's father had Tetsaiga passed on to Inuyasha because he knew the results that would come if his demon blood overpowered his human blood in life or death situations) Demons are much more blood thirsty than humans, paticularly those lower class and weaker demons who have not developed a higher order of thinking which is why they are often referred to a little more than "savage beasts". Sesshomaru has had centuries training and controlling and evolving those instincts and power with the supposed assistance and overview of both parents.
Koga too is very in tune with his wolf-ish animal instincts before he even gains his shards, and everyone else who does not seem to have major effects mentally seem to already know their place in the world and do not have questions on where they stand like Inuyasha who is constantly torn from his years of shunning in both races to wanting the jewel at all just so he can fit in to one category.
Now Kagome and the crew have thrown him for a huge loop with their ease in accepting every part of who he is now, which completely throws off everything he's learned about his kind in the eyes of just about everyone else in Feudal Japan. (And that's about 150 years of being rejected by everyone but his mom and Kikyo)
I believe it's mainly because it's those hatred and anger feelings which drive him into his full demon state in the first place.
This theme of transformation over anger is visible in many animes: Naruto, Bleach, Dragon Ball Z, etc.
The reason is because he's already angry/hateful, and the extra "evilness" the transformation gives him make him lose control entirely.
From what I understand (and it has been a while since I watched but its a fav of mine) no one other that Kagome or Kikyo are the only ones who can safely handle the shikon no tama without befoming corrupted. Kaede explains to Kagome early on in the first season that the Jewel will corrupt even men who mean only to use its power for good, not just demons; that only in Kaygome's position are the shards safe.
um Inuysha is half human and his father was one of the most powerful demons. Koga is just a wolf demon, I assume inuysha is much more powerful because of his father.
