I'll bet he was looking forward to the notoriety he would receive for what he did.
DELLA: Minor problem, Perry. That doesn't fit ANYTHING he said or did. No manifesto. No note. No hint in custody. Nada, zilch. Is this a problem, Perry?
MASON: Not at all, Della. Give me a week or so to think about it.
DELLA: Possibly, Perry, he was saving it all for a theatrical trial in which he would at last strut his stuff?
MASON: Nice work, Della. I like it.
DELLA: And yet, Perry, the operation was so fraught with risk that he couldn't reasonably have expected to survive for a trial. Is this a problem, Perry?
MASON: Not at all, Della. Shirley was actually Ted in a latex mask, the dog actually belonged to Bob, and it all stemmed from a confrontation between Shirley and Babs on a summer vacation in Istanbul back in college.
DELLA and DRAKE (in unison): Genius, Perry.
Roll the credits.
One of the psychological explanations for elaborate conspiracy theories is that believers need an assassination worthy of JFK, not a one-off by some "little commie punk" (Jackie).
John seems to be the mirror image of this: to be worthy of JFK, Oswald had to be an "evil monster," the Worst Person In the World.