BLOOMINGTON, Minn. — Two signs posted on the door of a nondescript dental office here asked passers-by to mourn the death of Cecil, a lion who was lured off his sanctuary and killed during a game hunt this month in Zimbabwe.

“WE ARE CECIL,” one read; “#CatLivesMatter,” read another. Nearby was a sign with a darker message for the dentist who said he killed the cat: “ROT IN HELL.”

In the hours since Dr. Walter J. Palmer apologized for killing the lion, he has gone from a dentist and longtime hunting enthusiast to a villain at the center of a firestorm over the ethics of big-game trophy hunting.

“I had no idea that the lion I took was a known, local favorite, was collared and part of a study, until the end of the hunt,” Dr. Palmer said in a statement. “I relied on the expertise of my local professional guides to ensure a legal hunt.”