The older Paul bro detailed his fear in the new Netflix doc, "Untold: Jake Paul the Problem Child" ... where he explained the backlash Jake faced after a multitude of controversies had him at an all-time low.

"I mean, both my dad and I felt worried for my brother's life," Logan said. "Jake's always been pretty emotionally unpredictable, and volatile."

"There was a point where we were both legitimately concerned that Jake was going to kill himself."

Jake was low-hanging fruit for internet trolls during the start of his YouTube fame ... getting backlash for wild pranks and outrageous stunts that led to pissed off neighbors and losing his role on Disney's "Bizaardvark," as well as lucrative endorsement deals.

"It was easy to rip me apart," Jake said. "It became a thing to hate Jake Paul."

"It's not easy to be hated by the whole entire world and it creates a very dark mental place."

But Jake found a new purpose through boxing ... which has led to increased popularity and respect from legends in the sport like Mike Tyson.

Jake has since racked up wins over former UFC stars like Ben Askren, Tyron Woodley and Anderson Silva ... and will enter the ring against Nate Diaz this weekend, where he looks to bounce back from the first loss of his career.