Why did Dr. Robotnik save Sonic in Sonic 2 for Game Gear

Robotnik more than less saw that having Sonic die just by falling in the lava was pointless to him, sure it would bring joy to him, but knowing that Sonic has been a pain in his side for a long time, he would want to kill him, so he "saved" Sonic from his doom and placed him next to one of his killer machines, thinking that he would be the one to kill him.