From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Van Zant is a deranged gunman who enjoys killing innocent people (apparently with the rationale that since it is "the end of the world", he wants to go out with a bang).
Dragon Ball Z
Van Zant shot an old lady and then made his friend, Smitty, shoot the old lady's husband (this scene is only shown in the uncut version). After he had killed many more innocent people, he decided to go to Fat Boo's house to try and kill him so that he could kill more people (which is ironic, as his excuse for killing people in the first place was because he figured that Fat Boo would destroy Earth). Van Zant shot Bee, Fat Boo's puppy, and then he and Smitty blew up his house with missiles. Mister Satan pummeled both Van Zant and Smitty. Van Zant later returned for revenge and shot Satan. Though both Bee and Satan survived their injuries, Fat Boo was so angry that, to control himself, he had to vent the evil within him out of the holes in his body. He was named Mister Boo after this event and the steam that came out of his head formed Evil Boo. The first thing Evil Boo did was blast Van Zant into nothingness. Van Zant shot Evil Boo moments before his death, but the bullets bounced off him (this scene is only shown in the uncut version).
Dragon Ball GT
Van Zant and Smitty appeared in a flashback that Mister Boo had when he bid farewell to Satan. He remembered Satan's bravery for standing up to them after Bee had been shot.
Van Zant appeared in Dragon Ball Z: Buu's Fury.