i want to know why edward cullen(twilight) came back from rebellious years ?

i am new here and i am not much of a Twilight fan but i like the character Edward Cullen (not only for looks) specially after lire Midnight Sun.....i know the real reason for Edward's return is remorse over murders he did but why did he feel that remorse?....he wasn't killing innocent and was actually doing a good job par finishing criminals so why did he left and came back to Cullens (not that i mind much)...i did not find much explanation of this in book so thought someone here might tell me....thanks in advance :-)