Since my best memory of Mr. Fitzgerald is from Going My Way, I was really rather disappointed
to find him to be the killer
.

Thank you! Very informative.

Artistic license, and the ability to use Mischa Auer, who was a fairly major character actor at the time. Since his specialty was playing Russians and pseudo-Russians, the character went from English Bright Young Thing to Russian prince-in-exile.
It's the same reason they made the judge Irish: they wanted to use Barry Fitzgerald who was a major, no "fairly" about it, star at the time.