The Art Scroll siddur brings down the word Echad (One) should remind us of two things when we say it in Shema. The first is that there is no G-d other than Hashem. The second is that though we see G-d displaying many contradicting traits, like kindness and anger, compassionate and strictness, He is still One and these traits are all in harmony. Although our human mind does not fully understand how these things can all be from One being, we need to realize that Hashem is so great that our minds cannot fully comprehend Him.