Why It Is Not Too Late To Repent And Turn To Christ

[Esquilax]

There were plenty of societies that burned people alive.

This is true, but they never represented the burning as a good act for the earthly life of the person being burnt. It was always a punishment, or a defense against something, never like "ooh, lucky you, being thrown into the fire!"

They did recognize it was bad, which was why they were doing it at all.

I was looking for objective grounding for morals for years now but I could never find it. Action can be objectively best for the certain set of values but values themselves are preferences which are subjective. For instance a person has opinion that burning people is good and my opinion is different. I will try to prevent him from burning people but it's just because I have different opinion. 1000 years ago many people had opinion that burning witches is cool and women were burned and everyone thought it was fine.
Besides the people feel burning if they are getting burned not if they burn other people

Just because you have differing opinions doesn't mean all of them carry equal moral weight; we do have a set of values by which we can measure the effects of any given moral choice- values which are themselves informed by what is best for a healthy society- through which we can determine those actions that are morally harmful and those that aren't.

In your example of a person who thinks burning people is good, sure, he can think that, but I can't envision a case that could be made for how burning people alive causes more good than harm. Meanwhile, the case explaining how it does more harm than good is easily visible.

Argument and evidence should be sufficient to sway one's moral judgments to one side or another, but you do need to have an argument or evidence for your position, or else you've just got a fiat assertion, which doesn't carry much inherent weight.