Professionals can be surprisingly sloppy.
I mean these guys are pros, right? https://en.wikipedia.org/wiki/Poisoning_of_Sergei_and_Yulia_...
And I always return to this story: https://en.wikipedia.org/wiki/Lillehammer_affair. If you can't count on the Mossad to have good opsec, who can you count on?
If anything, the impression that professionals do the job spotlessly seems like it goes hand-in-hand with the misattribution of this crime to a "professional."
From what's in the public record, even "professional" hitmen have their moments of conscience, laziness, procedural failures--and, apparently, quite frequently, presumably due to the moral pressure of conducting assassinations.
(Of course it's possible we don't hear about the hitmen who are truly pro-level, but I find that hard to believe.)