Atticus is known to be a decent man who always gives people the benefit of the doubt. The only problem is that Bob Ewell doesn't deserve that courtesy and he will only take advantage of the hesitation to kill or hurt Atticus or his kids.
The passage comes after Bob tried to kill Jem and Scout, having gotten incredibly drunk to get his courage up, luckily he is stopped by Boo Radley who emerged from his house for the first time in years to save them.
The quote simply makes it clear that Bob Ewell is beneath contempt and not deserving of normal human kindness because he has become so twisted and evil in his hatred.
This quote appears in Chapter Twenty-Nine of To Kill a Mockingbird after Bob Ewell has attacked Jem and Scout while they were walking home on Halloween night from the school pageant. In order to save the children, Boo Radley kills Ewell; Atticus does not initially realize that this is what happened and believes that Jem was the one to kill Ewell. He rushes to protect Jem, assuring the sheriff, Heck Tate, that Jem's actions were out of self-defense.
To Atticus's surprise, Tate insists that no one killed Ewell, and that Ewell instead fell upon his own knife. Tate comments on the terrible nature of Ewell's character, stating:
Mr. Finch, there's just some kind of men you have to shoot before you can say hidy to 'em. Even then, they ain't worth the bullet it takes to shoot 'em. Ewell 'as one of 'em.
In other words, Tate believes that Ewell's death was a necessary one in order to quell the impact that his horrible presence had on the community. To Tate, Bob Ewell was a complete waste of oxygen. It is also clear through this statement that Tate knows who really was responsible for Ewell's death and that he is ready to protect Boo out of an understanding that his actions were just.
This quotation means that Mr. Finch has a job on hand to shoot some people before they can escape.
The second part of the quotation: "Even then, they ain't worth the bullet it takes to shoot 'em" is a comparison of people who are to shot with the bullets.
The people whom Mr. Finch has to shoot are not even worth than a bullet. But Mr. Finch has to shoot them because they can escape if he doesnot shoot them.
It also says that the people are not even worth a bullet that Mr. Finch is using.
Hope the above information clarifies your doubt.