Everyone knows Goku is a good guy, but Dragon Ball's big hero has killed his share of enemies. Mercy and forgiveness are some of Goku's defining characteristics, almost to a fault. The Saiyan's human upbringing, combined with a well-timed bash on the head, ensured that the ruthlessness of his Saiyan blood was tempered and replaced by a kind and loving soul, albeit one who still loved to fight for the sake of competition. As a result, Goku has often made friends with former foes after besting them, with Piccolo, Vegeta and Majin Buu all coming to join the Z Warriors after taking a kicking from Kakarot, and even Frieza temporarily aligns with the good guys in Dragon Ball Super.
Even before getting to the making friends stage, Goku has a tendency to resist striking the killing blow; just knowing that the fight has been won is enough to satisfy the protagonist's hunger for combat. It was this lack of deadly intent that meant Frieza wasn't finished off as quickly as he might've been after Goku transformed into a Super Saiyan, and perhaps partly why Goku hoped Buu would be reincarnated, in addition to his desire for a strong rival. But despite his stellar reputation, Goku is no saint. Taking into account canon media and intentional deaths only, here's everyone Goku has killed in the Dragon Ball franchise.
Buyon - While fighting against the Red Ribbon Army, Goku encounters the monster Buyon, who General White keeps as a pet. The young Saiyan freezes Buyon into a block of ice after discovering his aversion to cold and then shatters him into pieces with a single strike. A cold-hearted death.
Captain Yellow - Goku punches this Tiger, also of the Red Ribbon Army, out of his plane while rescuing Upa. Given that there's no water around and Yellow never returns, he must've died upon hitting the ground, even though anime can be a bit flexible when it comes to surviving falls from a great height.
Officer Black - Revealed as the true final boss of the Red Ribbon Army, Staff Officer Black fights Goku inside a power suit, which Goku makes explode with the villain still inside. It should also be said that Goku appears to kill a fair few nameless Red Ribbon soldiers during this arc, but the numbers are almost impossible to verify.
Tambourine - After the King Piccolo arc brings with it the death of Goku's best friend Krillin (not for the last time, sadly), the Saiyan gets mean and decimates Tambourine with a Kamehameha.
Drum - Goku shows a similar lack of mercy to Drum, another of Piccolo's underlings. In this battle, the young warrior more or less kicks Drum's face off his head, with eyeballs shooting everywhere. Drilling the point home further, Drum's body is destroyed completely when the fight against Piccolo kicks off.
King Piccolo - Speaking of which, Goku kills the evil King Piccolo by punching straight through his body. King Piccolo was arguably the first traditional Dragon Ball villain in the series, and even though he would be reincarnated later, Goku's brutal victory set a bloody precedent that wasn't maintained.
Raditz - Possibly a tenuous entry, since Goku merely held Raditz still while Piccolo fired his Special Beam Cannon. However, that excuse is unlikely to hold up in court, so Raditz counts as a character Goku killed, especially since the whole strategy was deliberately planned out.
Yakon - After becoming more of a pacifist against Frieza and Cell, Goku found a way to indirectly kill Yakon, one of the beasts Babadi had the Z Warriors fight to collect energy for the revival of Majinn Buu. Goku intentionally powers up so much he overloads the beast with light and forces him to explode.
Buu - Perhaps the most understandable of Goku's kills, given how Buu was well on his way to destroying the universe. The Saiyan finally manages to eviscerate Kid Buu with his Spirit Bomb technique.
Frieza - Never one to make the same mistake twice (or 3 times, technically), Goku is forced to kill Frieza in Resurrection F after Vegeta stalls and Frieza destroys Earth. Whis rewinds time, allowing Goku to finish Dragon Ball's most infamous villain quickly.