Why did Buddhist start making idols of Buddha a few hundred years after Buddha died, even though Buddha himself said Idol worship was one of the biggest scourges of humanity
From personal experience as a chaplain:
I have found that when one worships the idol, and not the idea of that symbol, that is when the scourge to humanity issue arises.
Very much like mistaking the finger, when it is pointing at the moon, to be the moon.
Idols were created Kanishka's time. This was used as a tool to develop faith by worship.
even though Buddha himself said Idol worship was one of the biggest scourges of humanity
I do not thinks there is any reference to this in the Tripitaka.
to the best of my familiarity with the Canon, outside of clear instructions for laity regarding 3 ways of skillful bodily action
no killing no stealing no sexual misconduct
4 ways of skillful verbal action
no lying no divisive speech no harsh speech no idle chatter
and 3 ways of skillful mental action
no covetousness no ill-will no wrong views
expounded in the Cunda sutta (AN 10.176) in particular
the Buddha was rarely categorical about dos and don'ts, being rather a proponent of analytic approach, whos apt expression is found in the Vajjiya sutta (AN 10.94)
I don't say that all observances should be observed, nor do I day that all observances should not be observed...
If, when an observance is observed, unskillful qualities grow and skillful qualities wane, then I tell you that that sort of observance is not to be observed. But if, when an observance is observed, unskillful qualities wane and skillful qualities grow, then I tell you that that sort of observance is to be observed.
leaving it to our own discretion and judgement to decide what should and should not be done, necessary and unnecessary, useful and useless for our own good
going back to the Cunda sutta (AN 10.176)
These, Cunda, are the ten courses of skillful action. When a person is endowed with these ten courses of skillful action, then even if he gets up at the proper time from his bed and touches the earth, he is still pure. If he doesn't touch the earth, he is still pure. If he touches wet cow dung, he is still pure. If he doesn't touch wet cow dung, he is still pure.
Why is that? Because these ten courses of skillful action are pure and cause purity. Furthermore, as a result of being endowed with these ten courses of skillful action, [rebirth among] the devas is declared, [rebirth among] human beings is declared — that or any other good destination.
so in continuation of Buddha's line of exhortation it can be said that when a person is endowed with the 10 courses of skillful action, then if they don't worship idols, they're pure, and if they do worship idols they're still pure.