We always hear the terrible things done while he was in power, but was he known to do any bad stuff with his own two hands?
Not that we know of. It's possible, but I can't think of any source attesting to personal violence committed by him. He generally preferred to have others do violence on his behalf; for instance the murders during the Night of the Long Knives were committed when he wasn't around - quite deliberately so.
There would 4 "candidate times" for when he could have harmed or killed another human being.
When he was living bohemian in Vienna until 1913. Turn-of-the-century Vienna was a rough place to be poor, like all cities of the era. It's quite possible he got into a fight there and would presumably have had to hit someone. However, none of the sources on his pre-WWI life mention this, so it is pure speculation. It is worth noting that the lubricant of fisticuffs is alcohol, and Hitler had abstained from heavy drinking since an unpleasant experience in his youth.
The most obvious candidate time is during his service in the First World War. Hitler served as a regimental runner for the 16th Bavarian Reserve Regiment. The regiment took part in some very serious and nasty fighting, on one occasion being almost wiped out. However, Hitler's job as a regimental runner was relatively safe and to the rear - compared to most combat positions. To answer the question you posed in the comments, Hitler had no documented kills - he won his 2nd class Iron Cross for rescuing a wounded officer, and his 1st class Iron Cross basically for being around regimental HQ. If he did kill anyone in WWI, he did not mention it in later life.
The third candidate time would be during the chaotic Weimar years, when the Nazi party was routinely involved in street violence, and of course the Beer Hall coup in 1923. However, again there is no documented evidence of his taking a direct part in violence, aside from participation in the Beer Hall coup's climactic - but one-sided - gunfight on the Odeonsplatz in Munich in which four police officers and sixteen Nazis died. There's no evidence Hitler fired any of the fatal shots.
Geli Raubal was Hitler's half-niece, and stayed with him in Munich from 1929 until her death in 1931, and she had some kind of relationship with him - or at least he considered her "the only women he'd ever loved". Hitler was notably possessive and abusive towards her, keeping her a virtual prisoner in his house and having her tailed when she did leave. However, no sexual relationship was ever proven. When Hitler refused to allow her to travel to Vienna for singing lessons, there was an almighty row, following which Rabaul apparently shot herself with his pistol. Immediately, it was speculated that Hitler had murdered her. However, Ian Kershaw, Hitler's definitive biographer, dismisses such stories as contemporary anti-Hitler propaganda. Hitler was extremely upset, shutting himself away for days, however that doesn't necessarily prove anything. The theory is fairly durable.
TL;DR: There is a persistent theory that he murdered his niece and possibly lover, but otherwise there is no evidence that Hitler ever personally harmed or killed anyone.
EDIT: A couple of people have asked about the "unpleasant experience" that led him to quit heavy drinking. He got blackout drunk as a teenager to celebrate passing some exam and being given a school report card. He was discovered by a milkmaid the next morning lying unconscious on the road from Steyr to Garsten, having used the report card as toilet paper - he relates this anecdote in the Table Talk. He found the experience humiliating.
Current count of people who think they are being funny by noting that Hitler killed himself: 5
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I suspect he would have mentioned it in Mein Kampf if he did. Hitler was a message runner in any case after his regiment got hacked up at the battle of Ypres in 1914 fighting the French. Statistically, he is lucky to have survived that first year at the front. He was wounded by a shell and was also exposed to mustard gas at some point during the First World War and hospitalized. He was awarded the Iron Cross and considered a war hero. Post-war, he became a intelligence agent, pseudo recruiter for the army and refined his oratory skills. He was keeping tabs on the small German Workers Party and joined their cause and became their leader.
Hitler was very straight laced socially. He didn't drink alcohol, smoke, and was a vegetarian. He; like Himmler loved animals, particularly dogs. I'm not sure it would be right to dismiss him as crazy. I think he was in love with hate and power in a very sane way. He was passionate about raising Germany to greatness. I think he saw the world as pretty black and white through the lens of Aryan/German superiority. He saw non-Germans as sub-human garbage. He was able distance himself from the human cost of German lives (look at his actions toward the battles of Stalingrad or Berlin) and rationalize it as righteous payment to furthering the Third Reich.
He was a philosopher of hatred, did not love humanity, and was an accomplished liar.
what's the time scope of your question? He served in world war one but i'm assuming you're talking about only during his rise to power as a Nazi
Another (similar) response by /u/backmarkerS_E here
Didn't Hitler kill Ernst Rohm (during the "night of the long knives"), his once long-time friend whom the Nazi Heirarchy feared would one day challenge Hitler? I'm sure he personally killed him, or at least was around the SS guards when it happened. Historians, please correct me if I'm wrong.
During the night of the long knives, the book 'The Third Reich, a chronology' specifically calls out Hitler storming Rohm's room and arrested him personally. The book also references at this point Hitler 'taking up again the role of a political gangster' (or something to that effect, I don't have the book to hand, I'll look in the morning). You could obviously interpert this act as violence, even if no-one was hurt. Its also not out of the question he hurt someone whilst 'a political gangster
Followup question: Did Hitler kill any animals? Did he hunt, or oppose hunting, or anything? (I saw some sources say that some Nazis promoted animal welfare and opposed hunting, but I'm not sure about that.) Also, did Hitler personally kill his or Eva Braun's dogs before committing suicide?