This is an interesting question, because most people wouldn't even consider that Atticus Finch and Bob Ewell are similar. Their differences are many: Bob Ewell is racist and close-minded, poor, and violent. Atticus Finch is open-minded and always wants to do the right thing.
I would say that their similarities lie mostly on a superficial level. They are both white men who live in a southern town in America during very racist times. It is interesting to see that these similarities don't do anything to make them similar in character whatsoever. Thought they live in the same town, they do not share similar values as far as what is right and how people should be treated.
Beyond superficial similarities, Atticus Finch and Bob Ewell are similar in character in very few ways. One similarity, however, is that they are both human, and therefore both have weaknesses. Bob Ewell's weaknesses are perhaps more evident than Atticus's. Bob is poor and cannot provide for his family. Many of his actions against Tom Robinson could be a result of his desperate attempts to hold on to some of his pride. He wants, if nothing else, to have power over this black man who has done nothing wrong to him.
Atticus has weaknesses as well, but they may not seem quite as clear as Bob Ewell's. One of his weaknesses is that he wants to do what is right, sometimes to a fault. When Jem and Scout are attacked by Bob Ewell near the end of the novel, Atticus and the sheriff discover that it was Bob Ewell who saves them. At first, Atticus is determined to tell the truth, because he thinks this is what is right. What he doesn't realize is that he would be hurting the man who saved his children's lives. This is a weakness that Atticus has that makes him seem less perfect than readers may have otherwise thought, which actually serves to give him more depth as a character.
Your answer is very good but there is one thing, didn't Bob Ewell attack Scout and Jem instead of saving them? I got confused maybe you didn't meant to do that.
This is correct, Bob Ewell was the one who attacked Jem and Scout. Arthur Radley (Boo) was the one who saved the children.