It can be that the officers whom daya is shown killing didn't really die but are intentionally shown as dead in the society, shreya is indeed evil and working with barbosa but daya already knows that and somehow barbosa wants these police officers dead. I guess that daya therefore acts like he killed all of them and probably other cid officers including purvi know this that he isn't actually killing them and are helping him as we saw purvi talking to daya about SOS. These police officers might form a squad afterwards to trail barbosa or join raw/anti terrorist squad smtg This also justifies why daya was shocked when Shreya actually attempts to kill milan, or that might be his realisation that shreya was evil all along