Complex sentence: Later chroniclers have gone so far as to suggest that Sancho was a good king until his marriage, and his downfall was due to her bad influences, but such writings are biased by the need among his successors to justify his deposition.
Split into simpler sentences: Later chroniclers have gone so far as to suggest that Sancho was a good king until his marriage, and his subsequent difficulties were due to her bad influences. However, such writings are biased by the need among his successors to justify his deposition.