I haven't seen it cited anywhere (presumably because it's not on Westlaw or Lexis), and I don't believe I've seen other antebellum case like this from Pennsylvania, either. It's Commonwealth v. Crause, 3 A.L.J. 299, 303 (Pa. Ct. Oyer & Terminer 1846). Crause shot and killed a man who had unjustifiably attacked him; the court ultimately opined that this wasn't justifiable self-defense, because the attack didn't threaten serious harm (not a controversial legal principle at the time):

He [the decedent] had inflicted a blow upon his person. He had made no attempt upon his life. He used no weapon nor had he any weapon about him. There was no apparent danger of loss of life or of great bodily harm. There was no attempted felony upon his person, nor was there any threatened. The deceased was caught by one of the persons present. Those present in the house had interposed to prevent further violence upon the person of the prisoner. Where then was the necessity, this urgent necessity to take his life? The necessity must be a necessity founded in his own safety. It did not exist.

But the court also made opined that the defendant's being armed didn't by itself show that he was guilty of "willful, deliberate and premeditated" (and therefore first-degree) murder, partly based on the right to bear arms:

There is no proof that the prisoner was at this place to seek this quarrel, or that he did seek it. There is no proof that he was there to afford the deceased an opportunity to begin this quarrel with him, so that he might have an excuse for taking revenge upon him.

He had this weapon upon his person, it is true. It was heavily charged. But the citizens of this Commonwealth have a right to bear arms "in defence of themselves." This right is a constitutional right, and one which, "shall not be questioned."

Therefore, taking the act done, and all the circumstances which attended it, was the killing wilful, deliberate and premeditated? If it was not, the prisoner is not guilty of murder of the first degree. If it was, then he is guilty of murder of the first degree.

The author of the opinion was Judge John Banks, who was apparently a moderately prominent Pennsylvania official of the time: He had run for Governor in 1841 (getting 45% of the vote) and in 1847 became Pennsylvania Treasurer.