Majin Goten is indeed another slave of the evil wizard Babidi. He was the first Z Fighter that Babidi took over without any problem at all. Since Babidi took over quite easily, he was able to conduct more power into the little boy, making him Majin Goten. Plus, Babidi's extra power granted him Super Saiyan strength at normal form. He also has access to Super Saiyan 2, but only while in Majin form. Majin Goten is killed by his best friend, Trunks.
"You're weak now."
— Super Majin Goten to the hurt SSJ Trunks
Goten and Trunks were out in the mountains that day, to take a good look of nature. They went unattended, so there was nobody in charge. Back in Hell, Babidi used this as the perfect oppurtunity to strike. He analyzed the boys' powers and found that Trunk's was bigger. However he analyzed that his mental capabilities exceeded that of Goten's making him harder to control. Also, Babidi couldn't channel much power into him, rendering him under less control. Goten on the other hand was innocent and fun-loving, and that's an extremely vulnerable target to Babidi. He then took over Goten's mind quite easily and huge pulses of power into his brian, putting the little boy helplessly under his control. All those shockwaves translated into power and settled on Goten, making him achieve the power of an experienced Super Saiyan just in his base form. Majin Goten then looked around his new surroundings and eyed his horror-stricken best friend. He smiled maliciously and took off to Hell, since his orders were to free his master. But Trunks knew what had happened and knew the only way to get his friend back was to kill Majin Goten. Though hesitant at first, Trunks started hitting the flying Majin Goten with Full-Power Energy Volley. Babidi laughed when his attacks bounced harmlessly off Goten, because of his new Majin powers and Super Saiyan strength. Trunks then understood his mistake and transformed into USSJ and started hitting Majin Goten. Now the attacks were multiplied 85x and he knocked Majin Goten off the sky. However, Goten had one object and that was to get to Babidi, so he ignored the blasts, which kept hurting him. Angered, Babidi gave him new orders, and Goten suddenly turned. Then he lunged at Trunks with speed that Trunks never saw in Goten before. The two began fighting, Super Majin Goten and ASSJ Trunks. Trunks took the lead and tried multiple times to kill Goten, but Goten somehow dodged and deflected everytime. Growing weary and scared that Trunks might kill him, Goten initiated a transformation into his ultimate form...
"Trunks, you should really quit and die easily. Or I'm going to kill you and eat your heart!"
— Ultra Majin Goten to the beaten up Trunks
Super Majin Goten transformed into Ultra Majin Goten, which is really a Super Saiyan 2 . Trunks was shocked at Goten's ability to transform into a Super Saiyan 2, and believed that he should've been the one to achieve it first. Babidi contacted him then, saying if he let himself fall under his control, he would have the ability to transform into a SSJ 2 and be even more powerful than Goten. Trunks almost let himself but stopped himself midway, saying he won't make the mistake his father made. He then said he'll killed Majin Goten and restore his friend. But the looks of that diminshed greatly when Trunks engaged the new Goten in battle. Trunks was helplessly outmatched by the new Sper Saiyan 2. Goten kept taunting him about he was a disgrace to the Saiyan heritage and deserved to die. Trunks was shocked that his friend would ever say something like that. But then he realized that it wasn't his friend no more. In an angry flurry, he powered up a Buster Canon attack and shot it at Ultra Majin Goten. The Majin flew right into it, but come out of it with minor damages. The Majin flew right into Trunks's chest, cracking his ribs. Trunks fell back, and powered down to his normal state. Majin Goten flew into the sky, preparing a Kamehameha wave to end Trunks. Before he shot the Ultra Kamehameha, Goten explained Babidi has given him a new order- to destroy Earth before coming to Hell. This pushed Trunks over the curb, and the young Saiyan Hybrid transformed into a Super Saiyan 2.
"Oh man! You're a SSJ 2 now..."
— A disappointed Ultra Majin Goten when SSJ 2 Trunks matches evenly with him
With Trunks now a Super Saiyan 2, he clashed with Ultra Majin Goten. They two matched evenly for awhile, but Trunks started to showcase his new power, and started pummeling Goten. The Majin faltered greatly, even with his new powers. Goten's form started failing him and he started getting thrashed around by Trunks. Trunks cockily said no matter how much Goten cheated to surpasss him, he never would. Goten's good form (though taken over, still exsisted) was filled with unbeilevable rage when he heard that,
and Babidi, using the rage as fuel, made Majin Goten transform further beyond his usual capacity.
"WHY DO YOU KEEP ON TRANSFORMING?!?! BUT I DON'T CARE, I'M STRONGER THAN YOU AND I'M GOING TO KILL YOU!!!"
— A throughly irritated Mega Majin Goten when Trunks reaches SSJ 3 stage
Goten initiated his transformation into an SSJ3 . At the absolute height of his power, Goten began thrashing the life of Trunks. Trunks merely asusmed that he would power down because an untrained kid coudn't hold the nighty powers of SSJ3 for so long. But after frying Trunks with a Mega Kamehameha, Goten revealed his Majin abilities allow him to stay as long as an experienced fighter would. He then began to slowly kill Trunks, inflincting all the pain he can on the fighter. After several Ultra Kamehamehas, Galick Guns , and Buster Cannons, the Super Saiyan 2 began to loose his power. Mega Majin Goten chuckled that it was actualy Trunks that couldn't hold the power of a SSJ2. He then calls Trunks an experienced twit and ascends into the sky. He keeps calling Trunks names, saying that it wouldn't really matter even if he transformed. But before he unleashed MAX-POWER Mega Kamehameha, he suddenly powered down to SSJ2, and then quickly to normal. Goten yelled in pain, and Trunks realized that Babidi was having trouble controlling the power of a SSJ3 too, and his inexperience made him loose focus and control, making Majin Goten suffer. Seeing his chance, Trunks drew upon all of his power and unleashed a Last Resort Attack, killing the Majin within Goten and restoring him to his normal state.