In the story about Acher and R. Meir which appears in the Midrash, Acher does do teshuva before he dies; the version in our Gemara is quite different. Any thoughts on why he does teshuva in the Midrash and not in the Gemara? My guess was that with the Gemara presumably being later the rabbis had some other motivations for not wanting to see Acher do teshuva at the very end.
The story in the Midrash, as recorded here by Tosfos in the name of the Yerushalmi, does not say clearly that Acher did Teshuvah before he died. Rather, it says merely that he cried before he died. Rebbi Meir judged him favorably and said that "it appears that he repented." It says there, though, that a flame descended from the heavens and burned his grave, and thus even if he did do Teshuvah, it was not a complete and absolute Teshuvah.
Mordecai