Good question. He was probably gambling on the possibility that nobody would see him up at the window because nobody would know where the shot came from.
"What I don't understand is how we're going to stay alive this winter."
He was counting on confusion from the killing, and then getting away disguised as the veteran again.
shareIt isn't mentioned in the movie, but in the book the Jackal plots his escape route, down a fire escape into an adjacent alley, on his first visit to the apartment he intends to use as a sniper's nest.
The point, I think, is that the movie didn't need to state that in so many words. Since the Jackal is shown to be a consummate planner, it's implied that he wouldn't have left the escape to chance, either.
I’ve wondered this myself. An extreme interpretation might be: once he made up his mind to go through with the assassination assignment, even though he knew his cover was blown, he accepted the reality that it was a suicide mission, successful or no. The film gives us no information as to his plans of escape.
However – given that The Jackal was a consummate professional and ace planner, he must have had something in mind as to getting away. We can probably give him the benefit of the doubt even considering the mistakes, more of improvisation and impulse, than planning, that he made along the way.
The Jackal is not up for a suicide mission. He is prepared to take risks and perhaps enjoys this, but is no kamikaze. He wants to carry it out and get away to spend the money. If the OAS could have dug up a fanatical French nationalist who was prepared to die to get De Gaulle, they would have done so, but they went the "professional" route.
The book says something about a back exit in the building but there is not really much explanation of how he would get away. My feeling is he would have had a few minutes only to get away if he had actually managed to shoot De Gaulle. The few minutes depending on them not knowing where the shot came from.
"Chicken soup - with a *beep* straw."