Given Routh's history of criminality and disturbed statements, it's feasible that he could have left such a note. His reported criminal activity in the US and attempts to recruit mercenaries in Ukraine are indicative of his state of mind, and should have tipped off several law enforcement agencies. While it may have been easier to shrug him off as an typical unstable individual, authorities should have registered him as a potential threat long ago.
With the US election just weeks away, it's very disconcerting that Biden's own justice department — the same one that claimed the transgender Nashville shooter's manifesto was too sensitive to release — has chosen to publish an inflammatory and extremist letter calling for the death of the Republican presidential candidate. It appears the Democrats are willing to publish incendiary manifestos so long as they pose a threat to their political opponents.