It still sounds much less probable than anybody who would do it with an agenda.
> the streets are wide and you have to run for about 100m to the closest hiding spot
Can you post a link to the location? Not being from there and reading about these details for the first time, it's hard for me to imagine "one of Stockholm's busiest corners" but with no place to hide for 100m? No buildings? No entrances to the buildings? No entrances to the combinations of the buildings where you enter in one block of houses but can magically appear wherever across, already dressed in new clothes? That's all very doable in the busy cities I know, especially if somebody already planned and verified one or two escape routes.
Additionally, there could be a logic behind using an old gun: an origin of an old gun could be harder to trace, it could have been never recorded or marked.
https://goo.gl/maps/TWwgVMsroFnSkNH89
Here is the hot dog stand the couple walked past. The owner thought he saw someone following them.
https://goo.gl/maps/QysTTnjg8ek9PiRy5
Murder scene. The killer ran into the street after shooting Palme and attempting to shoot his wife.
https://goo.gl/maps/biseEKdGTw9A3mzK6
The killer ran up this flight of stairs. Another indication that he wasn't a professional. The stairs were ice-covered and slippery so he would risk falling.
https://goo.gl/maps/DMqBznb8jVgTv4ci7
After the stairs, it is not known what route he took. According to most witness accounts, he ran straight. Some believe he ran to the left though, towards the cemetery and hid there.
So much has been written about the murder so I can't really add much of value. Here are a few facts that makes it obvious that murderer wasn't a professional - hence no conspiracy:
1. The murderer tried to shot Palme's wife from a distance of about 2 meters but missed. 2. The murder weapon was a loud and heavy revolver. Real assassins use (silenced) pistols. 3. The murderer kept the gun rather than disposing it which professional assassins do. 4. The deed took place in one of Stockholm's busiest corners on a Friday after salary (lot's of drunk people out). 5. No good escape routes. I walk past the corner many times per week. All the streets are wide and you have to run for about 100m to the closest hiding spot (a small cemetary). 6. OP was often without bodyguards. There would have been way better opportunities to assassinate him.
Roughly, this was the conclusion of the experts from the FBI, who steered the Swedish police away from the political conspiracy theory to the "lone gunman" one. People still cling on to the conspiracy theory though. I think it is easier for them to digest because then there was some "meaning" to the murder, rather than a drug addicted street-thug doing it for no reason.
Fun (sad) fact; in the 90's you could often see the main suspect of the murder Christer Petterson (subsequently released) sitting intoxicated on the subway or commuter rail. People of course avoided him.