Complex sentence: At the height of his career Smith was known as one of the NFL's most fearsome defensive players, his sack record attests to this, though his public demeanor suggested he was quite mild-mannered off the field.
Split into simpler sentences: At the height of his career Smith was known as one of the NFL's most fearsome defensive players. His sack record attests to this, though his public demeanor suggested he was quite mild-mannered off the field.