Bad Guys
Android #17
1. Absorbed by Cell, and then killed during the self-destruct attack because he was stuck inside the monster and couldn't escape.
2. Fused with another Android #17 in GT. Taken out by the Z soldiers though.
3. Killed by Trunks in the alternate timeline. Trunks returned and easily defeated the android.
Android #18
1. Killed by Trunks in the alternate timeline. Trunks parried a blitz of energy balls from her, then blasted her in the face, killing the android outright.
Android #19
1. Killed by Vegeta. Vegeta saved Goku from certain death, then went on to kill this overweight android with a Big Bang attack.
Android #20 (Dr Gero)
1. Killed by his creations - Androids #18 and #17 in the alternate timeline from whence Mirai Trunks came from.
2. Killed by #17. His head is crushed by his own creation.
Brolli*
1. Killed by a triple KameHameHa from Goten, Gohan, and (In either his spirit or physical form, it was never revealed) Goku.
2. Cloned, then killed by Goten and Trunks, with them using an arsenal of attacks, including powerful KameHameHa's, exploiting his weaknesses.
Buu
1. Killed by Goku
Cell
1. Killed when he and Gohan matched KameHameHas. Gohan only managed a one-handed one, because the other was out of its socket. It was too powerful for Cell though, and overwhelmed him.
Cui
1. Killed by Vegeta. Underestimated the Saiyans power, and was annihilated.
Dodoria
1. Killed by Vegeta. He was beaten up, then supposedly shown mercy, but Vegeta blasted him as he flew off into the distance.
Dr Willow*
1. Body destroyed by avalanche. Remainder of him killed by a Goku spirit bomb.
Dr Kochen*
1. Killed by Dr Willow. He turned out to be a robot assistant, and plummeted to his death when willow opened up a crack in the floor by stomping on the ground, inadvertently killing Kochen.
Frieza
1. Killed by Trunks, who had come back from the future, just in time to stop Frieza and his father King Cold from destroying the Earth in revenge on Goku. He cut the evil being into many tiny pieces, then fried him.
2. Killed by Gohan (movie 12). He lunged forward, and buried his fist in Friezas gut, then blasted a hole through his stomach.
3. Future Frieza was killed by Goku. This happened in the alternate timeline that Trunks lived in.
Garlic Jr
1. Whilst not strictly killed, he was plunged into the Dead Zone, which was an alternative to death so-to-speak, by Gohan. This was in movie 1
2. Once again, hit into the deadzone by Gohan. This was for good though. This was from the series.
Ginyu Force
1. Jeice-Killed by Vegeta during the Frieza Saga. The fought for a while, then Vegeta finished him off with a Big Bang attack to the face.
2. Burter-Killed by Vegeta during the Frieza Saga. Goku beat him up, then Vegeta finished him off.
3. Guldo-Killed by Vegeta, just as he was about to destroy both Gohan and Krillen. Vegeta dashed in and released a powerful punch to his chest, followed by a beam that killed the Ginyu Force member.
4. Racoome-Killed by Vegeta during the Frieza Saga. Goku defeated him, then Vegeta finished him off.
5. Captain Ginyu-Technically killed by Goku when Captain Ginyu tried to change bodies with Vegeta, Goku threw a frog in the way at the last second. Captain Ginyu would then live the rest of his life as a frog from then on.
Janemba
1. Forced to transform by SSJ3 Goku. Killed in its most powerful state by Gogeta, with an energy beam right through it's stomach.
King Cold
1. Killed by Trunks. Came to Earth to cause suffering, but Trunks stopped him, blasted him in the stomach, then in the face, killing him in a painful manner.
2. Killed by Goku. This happened when he came to Earth in Future Trunks' alternate timeline.
King Vegeta
1. Killed by Frieza. He went in to try to kill the evil tyrant, but was easily defeated.
Kula*
1. He was pretty much killed by Goku when hit with a KameHameHa into the Sun. Everything died apart from his brain and a fragment of his skin. Whilst not strictly dead, as close as you could become.
2. Returned as cyborg Kula, only to be killed once again by Goku, who used a powerful energy ball (May have been a spirit bomb, I couldn't tell in my fansub).
Lord Slug*
1. Killed by Goku. He was distracted by Gohan's whistling, then finished off by the powerful Saiyan Goku.
Nappa
1. Killed by his own ally Vegeta after he lost the battle to Goku. Vegeta thought that he was a weakling, so he decided to dispose of him by throwing him into the air, and blasting him with a beam of awesome power.
2. Killed by Vegeta again in DBGT
Nappa's Saibamen
1. One self destructed to kill Yamcha. After being defeated in battle, it cowardly attached itself to Yamcha, and blew them both up.
2. Three Were killed by Krillen. He released an awesome blast that separated into many different energy balls, and rained down on them, with one getting away.
3. The one that escaped Krillen's blast was killed by Piccolo. It went to attack Gohan, but the Namek stepped in, grabbed its arm, threw it in the air, then it with a breath blast.
Piccolo Daimao
1. Killed by Goku. When fighting the Saiyan, he was attempting to kill him, but was defeated from Goku punching right through his chest.
Piccolo Daimao's Henchmen
1. All killed by Goku except one, who was killed by a hungry Yajirobe
Raditz
1. Killed by Piccolo while Goku held him The beam ripped right through his, and his brothers chest.
Red Ribbon Soldiers
1. Countless numbers of them were killed by Goku in Dragonball
Spice Boys
1. Mustard - Killed by Gohan. Beat up Gohan's friend, Krillen, and was punished by getting killed with a Masenko.
2. Salt - Killed by Gohan. Angered Gohan so much that he lashed out, and blasted him with a Masenko.
3. Spice - Killed by Gohan. Got the kid mad by beating up Krillen, and paid the price by getting Masenkoed.
4. Vinegar - Killed by Gohan. Got the kid mad and was blasted with a Masenko.
Tao-Pie-Pie
1. Killed by Tien after coming back as a cyborg to try and avenge his previous defeat against Goku
Zarbon
1. Killed by Vegeta. He transformed, but was still unable to defeat the mighty Saiyan, getting blasted into a lake.