Consumer advocate Ralph Nader doesn't believe the Boeing 737 Max jet should ever fly again in the wake of two deadly crashes thought to be caused by a system designed to prevent the plane from crashing.

"The problem is, they made a very bad, unstable design decision with the 737 Max. It's been criticized by many aerospace experts, by ex-Boeing people, and they just gotta take their losses," Nader said on CNBC Tuesday.

"You can't have a complex software system that tries to anticipate millions of configurations and expect that even if it doesn't have glitches and stitches, expect it to guide the pilots. The pilots are in charge of the plane, you can't have software with the potential to overpower the pilots and take the control of the plane from the pilots themselves."

Nader was then asked what Boeing engineers can do to fix the problem, which is rooted in an anti-stall system that was suspected of causing the crash of a Lion Air flight last October and an Ethiopian Airlines flight in March. The plane is grounded worldwide, which has affected Boeing's finances.

"The plane cannot be refixed. It has to be recalled and grounded and whatever they do with it, in terms of taking these engines off or reconfiguring it to a more stable plane … that's what they have to do," he said.

Nader, who ran for president in 1992, 1996, 2000, 2004, and 2008, also accused Boeing of focusing more on increasing its executives' salaries and its stock price and less on doing what's right, which would have been to engineer an entirely new plane rather than update the 737 to the Max.

Nader's effort against Boeing is personal, as his grandniece was killed in the Ethiopian Airlines crash.