The reason for having four gospels is so we have a more complete picture of Jesus' life. These gospels are telling the same story of one man's life (Jesus) from the perspective of four different people.
The Gospels of Matthew and John were the eyewitness account of two of Jesus' apostles. They saw all of these things for themselves, but obviously both of them weren't with Jesus ALL of the time. By including both accounts, Matthew shows us things that John didn't see, and vice-versa. The Gospel of Mark basically collects what Peter remembered, as he told it to Mark. Finally, the Gospel of Luke was a collection of many eyewitness accounts that Luke combined into one book.
The slight differences in views are easily explained by the personalities of the men writing them. For example, Matthew paints a much kinder picture of Judas Iscariot, implying that he was a good man led astray. John, on the other hand, called him "a liar and a thief." Just guessing, but I'd say Matthew probably got along better with Judas than John did, and the fact that Matthew (a tax collecter) was once considered a terrible sinner probably influenced his view of other "great sinners."
Anyway, you asked a good question, and I hope I gave a satisfactory answer. We have four gospels to have a more complete story, and the differences in attitude can be attributed to the personalities and viewpoints of the different writers.