OK, let's get the livestock folks involved here. I don't have a great deal of experience slaughtering lambs, but my experience watching it on TV is that if a lamb had digits and an opposable thumb, they would pull out a gun and fight, as they do struggle when they are confined preparatory to being killed.
The idea of going off "like a lamb to the slaughter" comes from their complete lack of fighting as they are taken to their final doom. Just another walk. They don't scream for a ram to come head butt someone.
Now this may seem silly, but it actually goes to the heart of the concept of trying to defend loved ones yet surrendering your own life. Joseph Smith was free, but surrendered himself to a highly biased, antagonistic representative of the Law who took him to a place known to be hostile to Joseph. He carried no weapon, put up no fight. In fact, he restrained the saints from trying to protect him.
That he was handed a gun, which he did fire and apparently wounded several of the attackers, who killed his brother. His brother had a gun in his pocket, which he did not use, nor did Joseph grab it and attempt to fire it. Apparently in the midst of the assault, he realized sacrificing his life, by going to the window, would draw the fire away from his two surviving friends.
Hmm, sacrificing oneself, without a weapon when one is loaded and available...
This was a martyrdom. He did fight to defend his friends, and then gave up when he realized his continued efforts would simply lead to the same fate as his brother. They would be killed too, and the story would not be accurately told about the death of the two leaders of the Church.
John Taylor did get the details about the death of some of the attackers at the hands of Joseph Smith, but he reports that it was something he was told, not had first hand knowledge.
I always find it amazing that the Mormons are accused of things like blood atonement and taking revenge, having Danites, etc. But what happens when their leader is killed? Nothing. Less than nothing. They don't even fight for their own homes. Within a year they would be driven out of the town they paid for with cash and blood, the sweat of many, the death of not a few in creating Nauvoo. Hundreds then die out on the plains in Winter Quarters (estimated at 500 people), and still the Mormons don't attack.
Joseph Smith, as a leader, was never known to attack those who attacked him. When Mormon leaders marched from Ohio to Missouri in 1834 because of abuses of members in Missouri, Joseph dissolved the troop before engaging in any contact or trouble. The lesson was not lost on the future leaders in the company of the Camp.
So remember, a lamb goes to the slaughter without a fight. But try to grab it and hold it to put a bullet in its head, and its going to put up a ruckus as best it can. It is the going part which is described in the phrase. We know that some of the early Christians allowed themselves to be arrested and fed to the lines because of their faith. But to say they didn't slap the lions on the nose as they ate them is just a fantasy. Those Christians are martyrs, even if they fought the moment of death with their hands. I suppose the occasional lucky gouge to the eye of a lion or working in groups to fight for as much life extension as possible or to protect loved ones does, by the standards of anti-Mormons, also disqualify those martyrs.
Seriously, was there any possibility at the time of the assault on Carthage Jail that Joseph and his comrades had any possibility they would survive? No, none. How did that come about?
The surrender by Joseph Smith to authorities whom he knew were going to kill him. We don't have any other statement of Joseph Smith saying he was going like a lamb to the slaughter. Just this time. And when he surrendered, he was unarmed, able to run, but resigned to his fate. So he surrendered to certain death without so much as raising his hand in defense.
Tell me again why this is not a martyrdom?