Very few aviation crashes are the result of a single failure. Usually, they are a combination of failures, all of which were required to produce the crash.
These crashes are no different:
1. the design shortcomings of MCAS
2. the refurbished AOA sensor on the LA airplane was not calibrated properly
3. LA put the plane back in service despite the failed AOA sensor
4. The crew of the LA flight were not informed of the failed AOA sensor
5. The flight crews did not follow the memory runaway trim procedure
All of these had to happen. A typical NTSB crash report dispassionately identifies all the contributing factors to an accident and issues corrective action recommendations for each of them.
I can't exactly say but a quick read comes up with some relevant points:
"According to the reports presented to the Indonesian parliament, a Florida-based aerospace company performed the repair of the faulty AOA sensor."
https://airlinerwatch.com/broken-aoa-sensor-in-lion-air-737-... [from april 2019]
So possibly it's not LA's fault, however
"On Nov. 5, CNN published an update on the crash investigation from Capt. Nurcahyo Utomo of Indonesia's National Transportation Safety Committee (KNKT) noting that the FDR review also concluded that the aircraft's airspeed indicator had been malfunctioning on four consecutive flights prior to the crash. Utomo also indicated the pilots should have recognized the malfunction when it occurred on flight JT610."
https://www.aviationtoday.com/2018/11/07/faa-issues-boeing-7... [from nov 2018]
“1. the design shortcomings of MCAS”
“5. The flight crews did not follow the memory runaway trim procedure”
How would the pilots have switched off MCAS if they didn't know it was there?
Of course many accidents have a "pilot error" component, the central issue is what made the pilots do the wrong thing. This needs to be analyzed on a systemic level, i.e. was the training good enough, did the pilots receive adequate information, did the aircraft system provide the pilots understandable feedback, etc.
For the pilots to do the right thing, they first need to identify the problem. That can be very difficult when a lot of visual and auditory alarms -- sometimes contradicting each other -- goes off at the same time. The aircraft manufacturer has to account for the human factors.
Boeing failed miserably in this regard: 1) MCAS didn't work as intended, 2) they deliberately left out essential info about the system from flight manuals. In addition the FAA failed in their oversight of Boeing.
The deaths are on them, not the pilots.