On fallout 3 deputy weld outside megaton was killed by a giant ant can this effect anything in my game

It can affect your ability to talk to Deputy Weld. Other than that, potentially it could compromise the safety of the thirsty old man that sits outside the city walls, or the caravans that visit daily. There are additional homeless men sitting outside of Rivet City and Tenpenny Tower if the initial homeless man is killed. The vendors have bodyguards, and the area in which they stop is generally tame. They should survive most attacks.

Simple answer: Not really.