Yes, I am fully aware of the difference between a felony and misdemeanor crime. However, larceny is considered a "breach of the peace" crime, and therefore, is subject to a citizen's arrest by one who observed the crime. Furthermore, one is required to effect such an arrest with the minimum amount of force necessary.
Here, you have a 70-something year old man attempting to detain a presumably healthy 28 yr old suspect. Seems to me that the disparity in force was such that the elderly gentleman had reasonable grounds to believe the younger man could easily resist said detention, and harm him in a serious manner. Thus, the gun was warranted, in my opinion. If the elderly gentleman had shot the subject, or even struck him with the gun, there would be an excessive force issue.
For the record, the lawful detention of a subject for a crime which was witnessed by a legally entitled citizen is a far, far cry from Sharia law. Holding one accountable for his actions is a cornerstone of this nation. Basic, fundamental laws, such as Don't Kill, Don't Rape, Don't Steal, must be upheld. Law Enforcement cannot be everywhere at every time, and its about time that people started pulling their heads out of the sand and realizing this.
If you do not feel that the theft of five gallons of gas that your neighbor paid for with money he sweated to earn is not worth arresting the subject, then that is completely your call. This gentleman felt otherwise, and in my opinion, was within the law in doing so. The sheriff's department apparently felt otherwise, and now it is in the hands of the Prosecutors and Judges assigned this case.