I can only state that Sareth was the second person to vote for Zannatos, right after Vavrek. Viking had pointed his finger of suspicion at Sareth, right before he was killed. Also, he makes the defense that he's the doctor, and as such, was innocent.
However, we don't know. There's also the unlikelyhood of having a cop, a doctor, and two matching character "couples" in a 7 person game, when everything was random.
Sareth's main arguement seemed to be that Vavrek was trying hard to lynch him. But as far as I know, Vavrek only posted twice about Sareth. Once to state that he bore close watching, and once here:
Vavrek wrote:I have told you before, there is no grand conspiracy between the elves and I. Were something to be arranged, it would not have been so public, so far from "behind closed doors" as yesterday's lynching of an innocent citizen. Viking-Sarine* and I merely both witnessed evidence and made evident our own bias when we spoke of our suspicion of you.
I did not dare fear that my bias would get such a reaffirming piece of evidence. Which is the greater fear? Knowing that out there, somewhere, are a pair of men intent on killing off this entire town... or that one finger-pointing elf is dead and the local
erranthalf-elf stands in the middle of town, repeatedly saying "I didn't do it!", after having killed hundreds, thousands, in the destruction of Emerylon? When we last met, you scared me out of my wits, asking simply where to find the elves. And now, it seems, you have found one...
*A viking version of Sarine, by the way, is just a fascinating concept.
In any case, not once did Vavrek ever point a finger of suspicion at Sareth, or vote for him. It could be that he just wanted to throw a little suspicion on Sareth in an attempt to prove that they weren't allies.
Also, what would be a better defense then lying about being the doctor? There would be no way to lynch him, since he would be such an important character. The perfect defense.
I say Sareth is lying about being the doctor. It's unlikely that we would have that many important roles and matching couples in such a small game, plus by lying about being the doctor, he's made the best choice to prevent himself from being lynched.