“The same source that a serial killer isn’t a tzaddik. What’s the source for that?”
Apparently, you think that it’s obvious that a serial killer can’t be a tzaddik and that this is proof that someone who does not keep Shabbos can’t be a tzaddik. So you obviously consider think there is a difference between the two (since you were assuming that it would be obvious that the serial killer can’t be a tzaddik even if the mechalel Shaboos can be). Therefore, it doesn’t prove anything.
In any case, why are you putting the two in the same category? Is it because they are both aveiros? Are you trying to say that someone who does an aveira can’t be a tzaddik?