Mr. Ewell said "one down and about two more to go," meaning Atticus Finch or possibly his children.
When Mayella Ewell accuses Tom Robinson of rape, it is the talk of the town. It immediately puts her family into the spotlight. It is an uncomfortable place to be. They have a reputation that is not very considerate. They are poor, and live in a way that Atticus described as "like animals" (Ch. 3). Most of them were uneducated, including Mayella.
Tom Robinson is convicted, and while in jail he tries to escape and is shot. Naturally, the gossip mill wants Bob Ewell's opinion on this news.
Maycomb had lost no time in getting Mr. Ewell’s views on Tom’s demise and passing them along through that English Channel of gossip, Miss Stephanie Crawford. Miss Stephanie told Aunt Alexandra in Jem’s presence ... that Mr. Ewell said it made one down and about two more to go. (Ch. 25)
By this, he seems to mean that he will go after about three people involved in the trial. When Tom Robinson was convicted, it was still an insult to Ewell because the jury actually deliberated. Also, the trial itself was very embarrassing. Ewell's pride was hurt. He is a drunkard and a small man.
It was Miss Stephanie’s pleasure to tell us: this morning Mr. Bob Ewell stopped Atticus on the post office corner, spat in his face, and told him he’d get him if it took the rest of his life. (Ch. 23)
Since Bob Ewell says "two more to go" and we do not know who the two are, we can assume that he might have meant Atticus and someone else, since he did spit in Atticus's face. It could be Judge Taylor, or possibly Heck Tate, the sheriff. It might even be Mr. Underwood, since he wrote a scathing editorial in support of Tom Robinson's rights, comparing him to a songbird. However, another interpretation of "two more to go" is Jem and Scout, since they are an obvious pair and they are the two that Bob Ewell actually did attack. Of course, he was drunk when he attacked them.
We know that Bob Ewell may not necessarily be an evil person, just a racist. Jem explains it well. When Jem says he has figured out how things work in Maycomb, he notes that the Ewells are on the bottom of the hierarchy except for the blacks. The blacks are the only people they can feel superior too. He figures this is one of the reasons "the Ewells hate and despise the colored folks" (Ch. 23). So Ewell may not necessarily be a killer, as terrible as his threat sounds. Maybe spitting in Atticus's face was as far as he intended to go, and he is only hot air. Nonetheless, he did attack the children with a knife, drunk or not. That was real violence.
Since Boo Radley intervenes, it is hard to know exactly what happened that Halloween night. It is not entirely clear what Bob Ewell had in mind for Scout and Jem. He might have intended only to scare them. We do know that Boo Radley is a gentle soul, and the chances that he would attack and kill a man for no reason are slim to none. If he intervened, it was probably because the kids were in mortal danger.