Is there any reason that someone who has committed horrible sins in the past (for example, mass murder) could not become a Jew? Assume, of course, that the sinner/criminal is willing to do teshuvah for his actions. 

It would seem that, for example, certain historical figures placed themselves beyond the pale of ever being able to become a Jew, even if they had repented in their lifetimes. Is it correct that such people would have been categorically forbidden from becoming Jews? If so, at what level of sin is that line drawn?