Many of his best roles were bad guys, so we tend to remember those best. And that's just as well--because Karloff (though by all accounts a true gentleman in real life) could always create one heck of a bad guy.
And during the course of his career, the villainous version of Karloff got to go up against a couple of the best and most famous of the fictional detectives.
The first time was in Charlie Chan at the Opera (1936). Karloff plays an amnesiac opera singer who has recently escaped from an asylum. Soon after, people start turning up dead backstage during a performance. Karloff is there--using his knowledge of backstage passages and trap doors to stay hidden. But is he guilty of the murders--or is someone else using his presence to cover his or her own guilt?
Like all the Chan films featuring Warner Oland as the master detective and Keye Luke as his son Lee, Charlie Chan at the Opera has a strong story and great production values. It's a cracking good mystery with an appealing protagonist and (of course) a great villain. Karloff's portrayal of a mad man only barely hanging on to his last shreds of sanity is downright creepy, but also forces us to feel a level of sympathy for the man.