He's very upset with his boss Mr. Blackton and assumes the quickest way to the top is to murder his superior. However, before bringing down the axe he realizes he'd be easily caught and convicted. That evening, Koster goes to see a double feature at the theatre: "Marked for Murder" and "Trapped."
This is a pretty familiar alibi in fiction of the time - the murderer claims to be at the theatre while a movie he'd seen earlier is playing. The stubs will hopefully clinch his alibi, leaving him free to murder Mr. Blackton...and that's exactly what Koster does.
He's done it! The perfect crime! ...Uh, assuming the police don't try to match his fingerprints to the axe. Early the next morning the police come to see Koster and ask for his alibi when Blackton was killed. He produces the ticket stubs and starts to describe the double feature, only to be handcuffed. Koster slipped up in a way he couldn't have foreseen:
Minutes after he bought his second ticket! Extremely lucky and yet unlucky at the same time!