Making the Snells disappear would have brought down the thunder on Marty. Remember, Dell had a short leash with Marty as it was; he could have whacked him right away if there was any risk of it getting screwed up.
Yeah, except the thought was for Marty to tell Del that the snells was screwing up their money laundering. So, Del and the cartel would have made them disappear, not Marty.