John Warnock Hinckley Jr.* Three names and a suffix.
Most Americans have a middle name (sometimes even two or more).
Furthermore murderers succeed all the time and still get described by two names: Richard Rameriez, Ed Gein, David Berkowitz, Jeffrey Dahmer, Albert Fish, Carl Panzram, Dennis Rader, Gary Ridgeway, Ed Kemper, Richard Chase, just to name a few. They all killed plenty people and all have middle names but we only say two. John Wayne Gacy OTOH gets all 3.
As to why the middle is sometimes omitted? Your guess is as good as mine, but clearly the deciding factor isn't success or failure.
After reading the article he only donated to the other group after 2021. Maybe he was a Trump fan until he tried to steal an election then changed his political views.
But either way we have no clue yet of why he did what he did.
Paywalled so can't see what they say, but didn't he donate in Jan 2021? He would have been 17, and thus prior to his registration to vote as a Republican.
The timeline's not incompatible with him being a never trump Republican.