Going out alone at night is prohibited today because of demons?

 Pesachim (112b) Do not go out alone at night, the Gemara states that this is as it was taught in a baraita: One should not go out alone at night, neither on Tuesday nights nor on Shabbat nights, i.e., Friday nights, because the demon Agrat, daughter of Maḥalat, she and 180,000 angels of destruction go out at these times. And as each and every one of them has permission to destroy by itself, they are all the more dangerous when they go forth together.

The Gemara states: Initially, these demons were present every day. Once Agrat, daughter of Maḥalat, met Rabbi Ḥanina ben Dosa and said to him: Had they not announced about you in the Heavens: Be careful of Ḥanina and his Torah, I would have placed you in danger. He said to her: If I am considered important in Heaven, I decree upon you that you should never travel through inhabited places. She said to him: I beg you, leave me a little space. He left for her Shabbat nights and Tuesday nights.

And furthermore, once Agrat, daughter of Maḥalat met Abaye and said to him: Had they not announced about you in the Heavens: Be careful of Naḥmani, Abaye, and his Torah, I would have placed you in danger. He said to her: If I am considered important in Heaven, I decree upon you that you should never pass through inhabited places. The Gemara asks: But we see that, notwithstanding these anecdotes, demons do pass through inhabited areas. The Sages say in explanation: These demons

113a

are found on the paths [gazyata] near the city, as horses belonging to the demons flee along those paths, and the demons come to lead them away. Generally, however, demons do not enter inhabited places. 

דף על הדף פסחים דף קיב עמוד ב

הטורי אבן (חגיגה ג ב) כתב דלאחר שגזר אביי שלא יעבור בישוב, שוב מותר לצאת יחידי בלילה בלילי ד' ובלילי שבת, והא דשמטי סוסייהו לא שכיח ולא צריך לחשוש לזה. אמנם הרמ"א (יו"ד קע"ז סע' ה') פסק בהדיא דגם האידנא אסור לצאת יחידי בלילה.

בספר שמירת הגוף והנפש (סי' ק"ט סע' א') הביא מכתבו של הגר"ח קניבסקי שליט"א בזה"ל שאלתי ממרן החזון איש למה אין נזהרין מלעבור בין ב' דקלים, והשיב לי אביי כבר גירש את כל המזיקין. אולם עיין בשו"ע הרב (הל' שמירת הגוף והנפש סי' י' בקו"א אות א') שלאחר שגרשם אביי ולא שבק להו רווחא, אדרבא צריך יותר לחוש לגזייתא, והיינו טעמא דהרמ"א שאסר לצאת יחידי בכל לילה, ולא רק בליל ד' ושבת.