أحسن الله إليك Regarding the ahadith that did not hold to blame muslims who had killed heretics who insulted the prophet ﷺ, is it evidence that if a common man today killed someone who insulted the prophet ﷺ that he is sinless?

No. Carrying out the prescribed punishments belongs to the ruler and the courts, not to individuals, as Shaykh al-Fawzan said, because otherwise it leads to chaos, revenge killings and fitnah. The reports from the Prophet's time were by his permission or approval, which is itself the ruling of the authority.