How to make good zombies?

[Drakenfel]

I heard that you can make better quality zombies with more sins and virtues skulls but I also heard that you should treat it as a buried body ie. More virtues with little to no sins but have also heard that sins and virtues mean nothing and only the number itself is important. So who was right?

Also how to know what an already existing zombies quality is and if bad is there a way to get rid of them?

[EVIL Shenanigans]

The relevant statistic is 'efficiency', which directly correlates to the number of white skulls (virtues) the zombie has, and ignores the number of red skulls (sins). So, similar in most respects to your buried corpses, you want to do anything that adds white skulls (remove Blood / Fat, perform Gold / Silver / Lye / Glue injections), while avoiding anything that subtracts white skulls (cause a Surgeon Mistake, remove Flesh / Skin, perform Acid injection).

Lastly, the 'Important Parts' (Brain, Heart, Intestine) make up the 'core' skull count of any corpse, so you can take the time to swap in ideal parts and swap out less-than-ideal parts. While this can be done to some degree in the mid-game through trial and error, it becomes much easier during the late-game when a perk becomes available, allowing you to see the skull count that each body part provides.