The TU154 crew followed the late ATC instructions,even though the RA was telling them to do just the opposite.(so who is to blame for that?)
In all air accidents,no matter who triggers the chain of errors,its always upto the pilots to prevent the last hole in the swiss cheese from lining up.
And now they have technology to help them, in the shape of "flt envelope protection" ,TCAS,and EGPWS.
The job profile requires individuals whose "Thinking" and technical understanding is sound, and who are able to focus,and clearly and quickly prioritise tasks.
Most people given proper training/adequate experience(preferably on type)are upto the demands of the job.
However,like in any line of work a certain small percentage(which varies according to airline/work culture)are just not upto the task,and in an ideal situation should be "retrained"(or put on some other job if beyond retraining.)
In the Charkhi Dadri case the collision actually occured when the IL76 crew tried to climb back to the level cleared,having already busted it and "safely" descended below the reciprocal Saudia 747s level.
As a sad postscript the Swiss skyguide contoller was killed by the father of one of the children who died in the TU154.