Not sure the Orthodox should point to how a group treated people with varying Christologies as a reason why it's awful.
As a Miaphysite, I agree in principal (though Servetus was burned alive for denying the Trinity and a dispute about infant baptism, not Christology), but there's something about the account of Calvin and Servetus that I find particularly affecting. I'm a historian by training, so I'm not unaccustomed to graphic firsthand accounts of humans being horrible to one another, but the Calvin/Servetus thing is one I find particularly gut-wrenching.
First of all, Calvin was out to get this guy for (partly) personal reasons from the word go. The two had been corresponding about the nature of the Trinity and other issues for awhile and Calvin didn't like it that Servetus had to gall to correct him (in a very snide way) on more than a few occassions. It wasn't just theological, Calvin seemed to take personal offense. A full seven years before Servetus ever set foot in Geneva Calvin wrote to his buddy William Farel:
If he [Servetus] comes [to Geneva], I shall never let him go out alive if my authority has weight.
He definitely had it in for the guy, and the premeditation makes what eventually happened all the more creepy. When Servetus, fleeing the Inquisition, made the mistake of stopping in Geneva and attending one of Calvin's sermons, he was seized by Calvin's cohorts. Once Calvin had this man in his power, he seemed to make a point of getting him back for his perceived slights. He had Servetus locked in a cramped, dank dungeon with no light, heat, or sanitary facilities and hardly fed him at all.
The accounts of the actual execution are particularly pathetic:
When Servetus, on the following morning, heard of the unexpected sentence of death, he was horror-struck and behaved like a madman. He uttered groans, and cried aloud in Spanish, "Mercy, mercy!"
The venerable old Farel visited him in the prison at seven in the morning, and remained with him till the hour of his death. He tried to convince him of his error. Servetus asked him to quote a single Scripture passage where Christ was called "Son of God" before his incarnation. Farel could not satisfy him. He brought about an interview with Calvin, of which the latter gives us an account. Servetus, proud as he was, humbly asked his pardon. Calvin protested that be had never pursued any personal quarrel against him. "Sixteen years ago," he said, "I spared no pains at Paris to gain you to our Lord. You then shunned the light. I did not cease to exhort you by letters, but all in vain. You have heaped upon me I know not how much fury rather than anger. But as to the rest, I pass by what concerns myself. Think rather of crying for mercy to God whom you have blasphemed." This address had no more effect than the exhortation of Farel, and Calvin left the room in obedience, as he says, to St. Paul’s order (Tit. 3:10, 11), to withdraw from a self-condemned heretic. Servetus appeared as mild and humble as he had been bold and arrogant, but did not change his conviction.
At eleven o’clock on the 27th of October, Servetus was led from the prison to the gates of the City Hall, to hear the sentence read from the balcony by the Lord Syndic Darlod. When he heard the last words, he fell on his knees and exclaimed: "The sword! in mercy! and not fire! Or I may lose my soul in despair." He protested that if he had sinned, it was through ignorance. Farel raised him up and said: "Confess thy crime, and God will have mercy on your soul." Servetus replied:, I am not guilty; I have not merited death." Then he smote his breast, invoked God for pardon, confessed Christ as his Saviour, and besought God to pardon his accusers.
On the short journey to the place of execution, Farel again attempted to obtain a confession, but Servetus was silent. He showed the courage and consistency of a martyr in these last awful moments.
Champel is a little bill south of Geneva with a fine view on one of the loveliest paradises of nature. There was prepared a funeral pile hidden in part by the autumnal leaves of the oak trees. The Lord Lieutenant and the herald on horseback, both arrayed in the insignia of their office, arrive with the doomed man and the old pastor, followed by a small procession of spectators. Farel invites Servetus to solicit the prayers of the people and to unite his prayers with theirs. Servetus obeys in silence. The executioner fastens him by iron chains to the stake amidst the fagots, puts a crown of leaves covered with sulphur on his head, and binds his book by his side. The sight of the flaming torch extorts from him a piercing shriek of "misericordias" in his native tongue. The spectators fall back with a shudder. The flames soon reach him and consume his mortal frame in the forty-fourth year of his fitful life. In the last moment he is heard to pray, in smoke and agony, with a loud voice: "Jesus Christ, thou Son of the eternal God, have mercy upon me!"
Farel walked beside the condemned man, and kept up a constant barrage of words, in complete insensitivity to what Servetus might be feeling. All he had in mind was to extort from the prisoner an acknowledgement [sic] of his theological error -- a shocking example of the soulless cure of souls. After some minutes of this, Servetus ceased making any reply and prayed quietly to himself. When they arrived at the place of execution, Farel announced to the watching crowd: 'Here you see what power Satan possesses when he has a man in his power. This man is a scholar of distinction, and he perhaps believed he was acting rightly. But now Satan possesses him completely, as he might possess you, should you fall into his traps.' http://www.ccel.org/s/schaff/history/8_ch16.htm
When the executioner began his work, Servetus whispered with trembling voice: 'Oh God, Oh God!' The thwarted Farel snapped at him: 'Have you nothing else to say?' This time Servetus replied to him: 'What else might I do, but speak of God!' Thereupon he was lifted onto the pyre and chained to the stake. A wreath strewn with sulfur was placed on his head. When the faggots were ignited, a piercing cry of horror broke from him. 'Mercy, mercy!' he cried. For more than half an hour the horrible agony continued, for the pyre had been made of half-green wood, which burned slowly. 'Jesus, Son of the eternal God, have mercy on me,' the tormented man cried from the midst of the flames ....
Like I said, I've read lots of horrible stuff, but this story always got to me. Something about the poor guy going mad anticipating his fate, begging for the sword instead of fire to no avail, the Calvinists putting a wreath of sulfur on his head and half-green wood around his feet to prolong the torture (it took more than 30 minutes to actually kill him in this way) and the callous, self-righteous attitude of Calvin, Farel, et cetera. I'm not pointing to this account as a reason to disprove Calvinism - it falls apart on its own easily enough without that - I'm just saying that this story is truly scarier than anything Hollywood has ever dreamed up for a Halloween movie marathon. I stand by that.