0 Cloverfield Lane is fundamentally about domestic abuse. Howard is a classic abuser, to such a degree that his actions run down a straight checklist of common tactics and warning signs. From his first moments with Michelle, hes more interested in controlling her than comforting her. He has no empathy for her, or understanding of whats going on in her head. He threatens her with violence when she disobeys his arbitrary rules, then seems baffled a moment later about why shes upset. Hes jealous and volatile. He terrifies her, then blames her for hurting his feelings by not showing him enough gratitude and respect. He isolates her from her friends and family, both physically, by locking her into the bunker, and emotionally, by repeatedly claiming theyre all dead and theres no way to even attempt to contact them. (Given that he doesnt actually know the nature or extent of the attack on the world above, this is one of the earliest signs that hes nowhere near the altruist he keeps claiming to be.)...
...When Michelle escapes the bunker and finds a new threat waiting, this is partially an extension of the abuse metaphor. For victims of domestic abuse, just getting out of the house doesnt immediately solve all their problems. For the metaphor to stay sound, 10 Cloverfield Lane needs to acknowledge that finding the courage to leave an abuser doesnt guarantee a happily-ever-after. For a moment, when Michelle first removes her makeshift gas mask and learns that Howard was wrong about the poisonous air, it seems like the movie might end on a note of relief, and the promise that her problems are over. But that would be facile, and would also mean that Michelle had been in a standard slasher movie, where arbitrarily bad things happen to random people, and nothing much is learned. And that wouldnt be in keeping with the movies actual arc, which is all about the way Michelle comes to terms with her abuse.
Michelles problems didnt start with Howard, and they dont end with him. They arent imaginary, like the toxic threat, and they arent just part of some vague general calamity. Theyre specific and personal, and they require a specific, personal catharsis. And thats the primary reason the big, direct confrontation is necessary in the final act.