When police arrived at Prinzregentenplatz 16 in Munich on the afternoon of the September 23rd, 1931, they found a young woman lying in the pool of her own blood. She was quickly identified as Geli Raubal, Adolph Hitler’s niece. The apartment in Prinzregentenplatz 16 was registered to Hitler and the gun lying next to the dead body was also his. The rumors of murder after a lovers’ spat started circulating Munich almost immediately.
Angela Maria “Geli” Raubal was a daughter of Adolf’s half-sister, Angela. Her father died when Geli Raubal was three years old. She, together with her mother and brother, moved to Hitler’s Berghof villa near Berchtesgaden in 1928, where Angela started working as a housekeeper. Angela was the only relative Hitler kept in touch with and also the only of his siblings he mentioned in Mein Kampf. Angela was visiting him while he was in prison in the 1920s and it would seem that they kept their relationship cordial afterward. Perhaps this is one of the reasons why Hitler adored Geli.

Hitler soon took fancy of young Geli and she moved to his apartment in Munich. Although it is unclear whether their relationship was intimate, it was definitely a romantic one. Hitler enjoyed her company and the couple was often seen visiting Munich posh establishment, like restaurants and theaters.
Both Hitler and Geli had a jealous streak and fights were a frequent occurrence in Prinzregentenplatz. Geli was accusing Adolf of having an affair with a 17-year old model named Eva Braun. Hitler, not to be outdone, discovered that Geli was secretly dating his chauffeur, Emil Maurice. After this, Maurice was fired and the relationship ended abruptly.

While Hitler’s part of the life that includes relationships and intimacy with women remains a mystery to this day, there are some accounts of his activities in this field with Geli Raubal. Wilhelm Stocker, an SA officer, wrote about one of them: ‘She admitted to me that at times Hitler made her do things in the privacy of her room that sickened her but when I asked her why she didn’t refuse to do them she just shrugged and said that she didn’t want to lose him to some woman that would do what he wanted.’
Apparently, living with Hitler proved to be too much for Geli, and she was desperate to regain her freedom. After the Maurice incident, Adolf watched her like a hawk and forbade her visits from her male friends altogether, just to be on the safe side. Geli planned to escape to Vienna and study music there, something Hitler vehemently opposed.
She was never to leave the apartment alone and was always accompanied by either Hitler or a member of his entourage. It was revealed later that she met a man from Linz she wanted to marry, but Hitler put a stop to that as well, trying to keep her all for himself.
On September 18, 1931, Hitler was scheduled to depart for Hamburg, after lunch with Geli. The lunch didn’t go as planned as the two got into an argument, which ended with Adolf storming out of the apartment. Tomorrow morning, he was recalled to Munich, as Geli Raubal’s dead body was discovered. The police ruled it as a suicide, committed with Hitler’s Walther pistol.

Hitler was devastated and have spent the next few days alone in a house at Tegernsee lake. He didn’t even attend the funeral but did visit the grave two days later.
The grief, however, was short-lived and he continued his relationship with Eva Braun as if nothing happened. Geli’s mother Angela, who was a strong opponent of this relationship, soon left Berghof and moved to Dresden. Hitler never forgave her objections.
Despite the claims made by Hitler’s political opponents, no further investigation into the death of Geli Raubal was made, and it remained officially a suicide. It is worth noting that the first police officer on scene was Heinrich Muller, who took the unfinished letter from Geli’s desk, which never made it to the evidence locker. Muller was later named the chief of the Gestapo and became one of the highest-ranking Third Reich officials.
Incidentally, he is the only senior Nazi official whose fate remains a mystery, as his body or evidence of his death were never found, yet no trace of him after the war was discovered. He was last seen in Hitler’s bunker on May 1st, 1945. After that, he fell off the face of the Earth.
There are several theories claiming that Hitler lost his temper and shot Geli in a fit of rage and left the apartment on September 18th only afterward, to provide an alibi. There is no evidence to support this claim and the fact that a note was found, but never shown to the public makes it highly doubtful one.
Source: historyinanhour.com