"I just think that even if they were to stay together for the rest of their lives, I think he would certainly cheat on her without a doubt," Bekah, 23, toldAccess Hollywood.
"I can't speak for Lauren, but I don't think that Arie is capable of having a monogamous, serious relationship... I really don't think that he's mature enough to handle a real committed relationship. I don't. He's impulsive and he doesn't think through his actions."
Arie and Lauren got engaged Tuesday night on After the Final Rose. He went running back to Lauren days after dumping Becca on-camera in early January.
If Bekah could describe Arie in five words after watching him break Becca's heart weeks after getting engaged in November 2017, she would choose "inconsiderate, selfish, psycho, childish and impulsive."
"I don't think he knows what he wants," Bekah continued, adding that Arie's ex-girlfriend was "spot on" when she tweeted before The Bachelor premiered that Arie would have trouble choosing one girl at the end.
Bekah certainly didn't hold back her feelings during her interview with Access Hollywood.
The spunky bachelorette noted, "Clearly, he has a track record of being a manipulative assh-le."
Bekah stands by the fact Arie never should have broken up with Becca on-camera, even with "producer pressure" weighing on his shoulders to make great, shocking television.
"The way he handled those interactions with Becca was so manipulative," Bekah said.
"I thought it was really emotionally manipulative. He wanted to see her break down I think, or he wanted something from her and he was not looking out for her feelings. He was looking for his own sense of well-being."
Bekah made similar comments during the After the Final Rose special on Tuesday. She said at the time Arie is "not a genuine person" and she hopes "Lauren gets out of that [relationship] as soon as possible."
"I don't want to see him or talk to him," Bekah snapped. "I mean, actually if given the opportunity, I'd probably have some choice words, but most of them would have to be bleeped out if it aired on television."
Bekah previously called Arie out on Twitter for direct messaging her on Instagram in early February when he was dating Lauren. Arie fans have argued his messages were just a harmless check-in, but Bekah feels otherwise.
"Everyone's been like, 'He was just being friendly, it was obvious that it was all light hearted,' but this is while the show is airing and while our relationship is playing back. He cried when I left. We had a serious connection and I don't think it's appropriate to be messaging me," Bekah told Access Hollywood.
"I guarantee if he was with Lauren, that Lauren didn't know about it. Because Becca didn't know he was talking to Lauren before he broke up with her!... I wasn't planning on putting him on blast until I saw how he handled things... I thought, 'No, he doesn't deserve to catch a break.'"