Per the article MCAS was originally intended to handle uncommon edge cases but was extended to cover additional (low speed) deficiencies. This expanded scope is what made MCAS as problematic as it is because it did away with the second input (accelerometer) and expanded the authority dramatically (from something like 0.6 degrees to 2.4 degrees of stabilizer movement).

The pilots knew something was going wrong. That wasn't the issue. The issue was that the bloody thing could mistrim the plane to the point of nigh irrecoverability, and no one knew enough about it until two planes full of people plunged out of the sky.

The plane may be able to fly just fine; but the way this thing was developed and brought into mainstream use had critical problems in terms of essential information being communicated.

All the decisions and motivations behind these lack of communication have to some point been traced back to trying to circumvent regulations in order to prop up share price by scoring sales of a new airframe of comparable efficiency to the a320neo.