Up until a few days before his death Hitler still believed Germany could push the Allies back out of Germany and he could still win the war. He was totally delusional and this delusion was fed by his officers being too scared to tell him the truth of what was happening outside the bunker. Hitler would go into a massive rage if given bad news and many feared if they told him the truth during one of these rages he would order them shot.
Hitler by early 1945 was not a well man with failing health. As a soldier during WW1 Hitler contracted Syphilis in a sexual encounter which in later stages causes madness and by the mid 1940s was showing itself where Hitler was starting to get these severe anger rages losing sight of what Germany should be doing to win the war and was just interested in revenge attacks on countries he blames for ruining his plans of ruling the world. Hitler also was suffering Parkinson's disease and his hands shook uncontrollably.
Hitler realised a couple of days before his death that the war was lost and realised his officers had been lying to him. He realised all was lost as did other top Nazi leaders. Hitler had heard what happened to his fascist friend Mussolini in Italy where partisans had shot him and taken his body to a square and hung him upside down naked, and the Italian people angry that Mussolini had taken Italy to a war they didn't want abused, hit his body with sticks, and his body was shot a couple times as well. Hitler feared this would happen to him and decided to control his own death. He decided to commit suicide, and his mistress Eva after hearing Mussolini's mistress had been murdered and hung up next to him decided to suicide as well. Hitler and Eva decided to marry 2 days before they suicided and on the day of his death they went into a room and Eva took a cyanide tablet and died then Hitler put a gun to his head and shot himself. Hitler had left orders for officers to cremate their bodies and an officer was sent to find petrol to use to burn the body but could only find a couple jerry cans of it as the Germans had run out of virtually everything. The body was taken outside and set fire to but there was not enough petrol to destroy the body completely.
When the Russians finally defeated the Germans they demanded Hitler's body. The Germans gave it to them and it was taken back to Russia for forensic testing to be sure it was Hitler. After determining it was Hitler's body the Russians cut his body up and secretly sent it out to be dispersed in forests so it could never be retrieved by Nazi sympathisers and Hitler being put in a grave for Nazis to gather at.
There were reports that Hitler had managed to actually escape to Argentina which had a large sympathetic German ex-pat population and from time to time magazines showed pictures of Hitler sitting at cafes. These were all false as firstly Hitler was a very sick man back in Germany yet the men in the photos were healthy. If they had been Hitler they would have had body guards as there were still many loyal Nazis there who would have protected Hitler with body guards. Back in Germany Hitler was always surrounded by body guards. The other thing that showed that it was not Hitler was the fact the photographer did not follow the man home. If you had spotted the real Hitler at a café any smart person would have called in the Allies to catch him. It would have been the story for the century and the photographer would have become rich and famous for being the guy who found Hitler and helped get him arrested. But they just took a photo and made up some story to make and article for the magazine. Likely they found a guy that vaguely looked like Hitler and sat him at a café and took his photo.
Recently the little of Hitler's remains that were not sent away to be lost in the forest were tested with modern techniques and they could tell the man was a vegetarian, and with all the Russian forensics were able to come to a conclusion it really was from Hitler as in the final days of the war the chance of finding another man who was so close to Hitler's height, dental records, and had been a vegetarian as Hitler was, would have been incredibly unlikely. So I think we can accept Hitler did commit suicide in the Bunker as those there testified.