Obviously, you cannot be omniscient. The idea is that you have to accept that you made the best decision with the information you had at the time. If that information changes, make new decisions.

I am non smoker, never started.

My parents were smokers - can I blame them for their smoking habit that had negative effect on me, on their health?

Well not really because they never had a bad intent.

When they got smoking induced health issues were they able to make a new decision? - Not, it was too late.

Well, this is why "it boils down to" this. You can't control what other people do; you may have influence, sometimes, but never control. You can't blame people for what they do, if you live with the assumption that 'most' people are making the 'best' decisions with the information they have available at the time of making it; where 'best' means something that could be an entire book.

Every decision boils down to one of four types of decisions (philosophy 101):

1. inconsequential and routine (what color shirt to wear)

2. inconsequential and once-in-a-lifetime (what color shirt to wear _today_)

3. consequential and routine (which route to take to work)

4. consequential and once-in-a-lifetime (who to marry)

The thing is, 1 & 2 can be and usually are consequential; it's just that you lack the ability to detect the consequences. Usually. I met my wife, on the beach, with some friends. The decision that led there, I thought, was inconsequential at the time.

But yes, your information will always be imperfect. I have scars all over my body from doing things that should have been fine. I've nearly died and somehow escaped (or been rescued from) the situation too many times to count. You have to live with consequences and not all consequences are knowable in advance; and for that matter, not all consequences are bad.