The gemara goes through the possibilities of when we can judge a capital case, with the understanding that we require a ha'lanas hadin (sleeping on it), and to avoid inuy hadin. The gemara asks why can't we judge on Friday, announce the verdict on shabbos and carry out the punishment on shabbos? The gemara answers: אין רציחה דוחה את השבת. The gemara then goes on to prove why beis din cannot carry out capital punishment on shabbos, and ultimately concludes that the issur that would be violated with capital punishment doesn't push off shabbos.
Tosafos asks, how can the gemara suggest that the verdict should be issued on shabbos; woudn't this violate the mishna in beitzah 36a which says that there is an issur sh'vus to judge on shabbos? Tosafos answers that the problem with judging on shabbos only applies to monetary cases because it will lead to writing, but by a capital case this is not a concern.
Tosafos' question seems very strange. The gemara explains that issuing the verdict on shabbos would require us to carry out capital punishment on shabbos which we cannot do. The gemara seems to hold that it would be an issur d'oraysa to issue the verdict since it would inevitably cause either a inuy ha'din or violation of shabbos. Why does Tosafos sugggest that it should be assur based on a sh'vus d'rabonon that it may lead to writing? In truth, the Ran says exactly that to answer Tosafos' question - יש לומר דהא עדיפא ליה דאסור מדאורייתא כדילפינן ממושבותיכם וכו' לומר שאין מיתת ב"ד דוחה שבת, ומתניתין נמי דינא דאורייתא קתני. Being that there is a din d'oraysa that would make it assur, which is what the gemara actually says, why is Tosafos asking that we should say it is assur due to a violation of a sh'vus? Perhaps Tosafos is asking rhetorically to point out the the issur sh'vus only applies to dinei ma'monos and not dinei nefashos.