I thought he was justified in killing his friend since his 'friend' was going to kill him if he didn't go along with his friend's crime.

especially the way he nonchalantly .
shoots his friend at the start
That was a bit weird but only fair I suppose because John had no qualms about doing it to him first. In the earlier adaptations, least of all in the 1960 TV version for the Dow Mystery Hour, the doctor conspired WITH Fleming to fake his death so they could both search the house for the money, and even when the Bat murdered Fleming, the doctor had no idea he was dead.

Good Vincent Price scene that you described. I'm a huge Vincent Price fan. But I don't think that he has enough screen time to make it his film entirely.

I agree that he's a very urbane villain, but he certainly doesn't carry the movie. Though he's first billed, his character is not much more than a support for Cornelia Van Gorder and Andy Anderson.