I love how this story has spawned two camps of people calling for Palmer's head. You have the people that are against any type of hunting and/or trophy hunting whatsoever, and then there is the group that feels like Palmer made an illegal hunt and therefore needs to be held accountable.
For the first group of people I think that they are a non-entity to this story. Whether it was Cecil the lion or some unnamed deer in NY doesn't really matter... They'd be against the hunter regardless. Unfortunately so many of the people blowing this story out of proportion (compare this story to any story of a human being killed) tend to be in this camp. They see an animal (esp. a mammal) being killed and are immediately saddened and outraged by the lion being killed, whether or not the kill was legal. These people, while keeping the issue of hunting in general alive do nothing for this particular story, esp. since one of Zimbabwe's "tourist" industries is big-game hunting.
The second group of people I think have more to offer to this particular story. They don't condemn Palmer for being a hunter, but rather for making an illegal kill. With this group, there may be some that are calling for Palmer to face legal charges that he may not be guilty of, but at least these people have more of a rational response and so can learn if there are any errors in their judgement.
The most common misinformation that I have heard is that Palmer killed an endangered species.... While the Asian lion is indeed endangered, the African lion is not; at most one could consider the African species to be threatened. Thus as to the legality of killing this type of lion, Palmer is not guilty of any wrong-doing.
As for following the lead of the hunting guide, again many big-game hunting sites as well as other organizations interested in Afican hunting (eg lionwatch.org which is a site for the conservation of lions worldwide) have also echoed this sentiment. Thus a majority of the blame should lie with the guide for giving improper direction.
Where I would find fault with Palmer is in his actions after the kill, although considering the almost immediate hostility that was thrown at him I guess I'm a little more lenient. I felt that after the kill, Palmer must have known that the lion was special in some way. Even if he truly didn't see the radio collar at the time he was making the shot, once he got up to the body, he should have recognized it and then immediately reported the incident to the authorities (or at least to the US embassy which in turn could alert the Zimbabwe authorities). However by him leaving the country and now going into hiding here in the states make it look more like he knew it was wrong and was more afraid of getting caught. Had he turned in his guide immediately after he realized that his guide gave him advice that led to the illegal kill, then I think that more people would be understanding.