The ending doesn't seem probable.
The hunter assumed the tiger was the last of its kind. He didn't appear to have any evidence to support his assumption. He only saw the one tiger once and followed it for a short time before shooting. Wouldn't it be smarter to try to follow the animal as long as possible to see if it would lead him to where other tigers might be living?
He regretted taking the job and he didn't want the toxin to fall into the hands of his employers. Rather than killing the tiger, he could have captured it for study by the nearest university, zoo, research center, etc.