Hang on, is this one of those observational, Lateral thiinking things.
The guy wrote a note expressing a wish to kill himself.
But jumping off the building is not mentioned as a means to this end. He could have been a base jumper, wishing to die at some point, just not that day. Whilst base jumping, he realises that things are worth living for and writes a note to disqualify his previous note.
Then he is shot by a "stray" bullet...perhaps not a stray, Perhaps the shooter was carrying out the base jumpers wish to die and, that the latter note was due to be dispatched to the shooter to cancel the "hit".
Or perhaps the shooter had to kill the guy to gain some advantage, like inheritance which would only pass to him if the jumper was pronounced to have died whilst not being of sound mind...was the tower an lunatic asylum...having received a note confirming the intention to die, whilst sane, this would scupper the shooters plans.
death is always certain at all times....as is tax, so this statement, whilst made after the event of jumping, has no direct connection to the jump itself.
Or was our Jumper actually a professional stuntman, albeit a suicidal one, who was planning on killing himself later that day but was killed on the job instead