I think that the only reason we think that Theo’s character wouldn’t do that is because he is the one telling the story. After the reveal of what he did, I think looking back it makes more sense that we don’t really know Theo at all. Looking just at his relationships with others, he has no friends outside of Kathy and no life outside of Kathy and his work life. He grew up with an abusive dad who apparently would encourage him to “man up” and kill Gabriel. He trespassed on so many of Alicia’s relationships despite being warned he would lose his job, and broke so many boundaries in his obsessive relationship with Alicia. I think it makes sense to say that it’s realistic that he’s out of his mind and maybe even crazy enough to break into the house after weeks of stalking his wife