Except when claiming self defense it will be rare that the defendant will be able to effectively avoid testifying. He has at least the burden of producing evidence at least making a prima facie case of justification. His testimony might well be the best, if not the only, evidence of what and how things happened.
Physical evidence can be equivocal. It might not be able to tell the story -- only corroborate it. There may be no witnesses, and witness can often be unreliable.
A defendant claiming self defense might have a better chance of not needing to testify if the incident took place at his home. There one might expect there to be signs of forced entry which would help tell the story of what happened. But not all incidents take place at one's home.
Franks comments on my prior comments is getting afield of the topic... however to clarify from an experienced criminal prosecutor and defense attorney, smartly worded statements to the law enforcement before invoking can be elicited through the police and detectives through effective direct or cross examination, thereby eliminating any need to have the accused testify.
Q: And isn't it true Detective Johnson, that Mr. Smith told you that the deceased came at him with a knife in the parking lot?
A: Yes
Q: And Mr. Smith told you that he told the aggressor to stop at least three times?
A: Yes, that's what he said.
Q: But the man with a knife kept coming and didn't stop?
A: That was Mr. Smith's version.
Q: And he said he feared for his life so he drew his pistol?
A: Yes
Q: And he has a lawful concealed carry permit?
A: Yes
Q: And the requisite training?
A: Yes
Q: And the ammunition used was XYZ?
A: Yes
Q: And that's identical or very similar to what the police carry in their guns?
A: Yes
Q: And in your investigation, you determined that both shots were fired at about 10' distance, in rapid succession, while the deceased was standing?
A: Yes
Q: And when you talked to witness ABC, she told you that a version of events that is generally consistent with what Mr. Smith had told you?
A: Yes
Q: When you talked to Mr. Smith at the scene, he told you he acted in self defense?
A: Yes
Q: And he told you that he wanted to press charges against the attacker for assault?
A: Yes
You get the idea. The same would be done with the forensics scientists, other experts, witnesses, etc.
There would be little, if anything, an accused to add to this by his testimony, and many times accused people look shady when testifying, or just say REALLY DUMB things on the stand, and this can sink them. Call it nerves, or trying to impress, or being tricked into getting angry, etc.
An accused should rarely need to testify if the event was justified (forensics will most likely tell a clear story) and a person is intelligent about what he says to the police before invoking.
Sorry to take this afield, but it's an important understanding of criminal law.