I just re-watched the movie “Arsenic and Old Lace” and I was wondering why Mortimer was so singularly focused on having Teddy committed throughout the whole movie?
He always knew Teddy was crazy. But when Mortimer found the body he decided Teddy must be committed even though his aunts happily admitted to the murder(s). The whole movie that seems Mortimer’s singular goal.
Why?
As many times as I’ve watched the movie, I’m not sure. It drove the plot would be my answer.
I feel like he decided to commit Teddy so he could take the rap for the Aunts if it came to it???
Maybe he figured if Teddy wasn’t there to “dig the locks”, the old ladies would have to stop their hobby.
I think that Teddy needs asylum, in an Asylum, a safe place where they take care of crazy people, because (1), the Aunts are getting old and won’t be able to take care of Teddy forever, and (2) now the Aunts /certainly/ aren’t the right people to be taking care of Teddy, and (3) now we need to rescue Teddy by getting him committed before he’s arrested.
Coincidentally, I played Teddy in our local community theater a couple of months ago. That’s a delicious role.
Mortimer doesn’t want to turn his aunts over to the police. He says as much as soon as he finds the body, and they explain what’s going on. He wants to have Teddy committed, so that he can blame the murders on Teddy. Everyone in the neighborhood knows that Teddy is crazy, so he figures that will be easy for people to believe. He’s also thinking that he can convince the aunts to stop killing old men.
Would that work? Probably not; Abby and Martha are pretty deeply committed to their program of being merciful to their gentlemen (that is, killing them). But you’re not really supposed to think about it too much. It’s a farce, and the key to a farce is to keep things moving so that the audience doesn’t have time to think about it. The plotline of any farce will pretty much fall apart if you apply too much logic to it.
I played Jonathan Brewster, so I got to sit backstage through most of the play and watch the whole plot unfold. I’m pretty much in agreement with @MrAtoz.
Mortimer dearly loved his aunts. He also knew they were at least as nutty as Teddy, although a good deal less flamboyant. He probably calculated that Teddy would fit right in at Happy Dale, while Abby and Martha would die if they were removed from the home they’d lived in all their lives. He may also have thought the aunts would quit killing the old men if Teddy wasn’t around to bury them, but I wouldn’t read too deeply into hidden motivations.
Gabe? Is that you?
It would be funny if both of us were in the same production without realizing it! 
Edited to add: We actually talked about this a bit during rehearsal. Mortimer’s line to the aunts is “I can’t turn you over to the police!” Our Mortimer asked, “Why can’t I, exactly?”
Steve??? It’s good to hear from you after all these years. BTW I had some notes on your performance I was meaning to give you.
I have seen Capra’s film many times, but have only seen the original play only once, and have never read it. But based on my familiarity with Capra’s version, I agree completely with @MrAtoz.
I also want to say that after seeing the play, I realized that Capra cut a lot of (IMHO) dull development and exposition from the original text and transformed a mildly humorous play into a madcap comedy classic.
I was about to say that it’s a shame that we’ll never see Karloff as Jonathan, but then I learned that he reprised the role he originated on Broadway in a 1962 TV production, with Tony Randall as Mortimer. And the whole thing is on YouTube!
I haven’t had a chance to watch it yet, but here it is:
ETA: I substituted a link to what seems to be a better video than my original link.
Just leave them in the green room, and I’ll look them over. 
So did I. I’ve never watched the movie (that I can remember), but at least for the play, I also agree with @MrAtoz’s response. But also, @Melbourne also makes a good point about Teddy needing someone to take care of him. If the aunts were arrested, what would happen to Teddy?
I can barely remember the movie, reading the play, or a high school performance I attended. My vague remaining impression is Mortimer didn’t want his aunts foul play to be found out and Teddy was the obvious patsy. Beyond that I didn’t question the premise.
My grandfather played Teddy in some performance long before I was born. Mom kept the Pince Nez glasses he wore for a long time. Don’t know what happened to them.
That was a fun watch. AT 56:10 Mortimer explicitly says “If I can make him [Teddy] responsible for those twelve graves I can protect you!” Which agrees with the motivations everybody was suspecting.
Teddy might have gone on safari and roughed it for his remaining years, but Mortimer had that covered. Have Teddy committed to Happy Dale Sanitarium where they look after him, and the aunts can stay home where they’ll be comfortable and safe. It’s all very logical for a screwball comedy.