The IRB is trying to crack down on spear tackles in the sport. The rules state that if you lift a player up off the ground you are responsible for ensuring he comes back down safely, the standard with spear tackles being applied in regards to rotating the player past horizontal. Intention or malice (or lack thereof), according to the IRB's official stance, are neither mitigating nor aggravating circumstances.
That said, I believe that if you lift a player off the ground and rotate him past the horizontal point, you're going to get a card, and it only becomes a question of red or yellow. If you realize your mistake and let him go, then you can get away with a yellow. If you drive him into the ground, upper body first, or drop him from a dangerous height, you should get a red. That's why last night's ruling is open for interpretation, because he did not drive him into the ground. However, he was up pretty high in the air, and while I do not believe it was malicious (I don't believe anyone thought it was), it was obvious that he was intentionally rotating the player, though too far because he mistimed it.
I believe, had he not let him go, and fallen to the ground with him, he would have gotten a yellow instead, but it was a pretty reckless tackle from the beginning. He set himself up for a bit of a gray area between yellow and red. The sending off was arguably harsh, however, if he had gotten only a yellow card instead, everyone would probably be talking about how he was lucky to get away with not getting sent off.