Bad as in why they fell or bad as in general? Try this:
Aztec: ruthless rulers (led to much dissension between neighboring tribes), no wheel was introduced (technology was stunted), and there was no written word to record history
Inca: empire stretched across South America by the end of their reign, thus stretching people and supplies thin for Incan rulers, and as with the Aztecs, there was no written word and no wheel technology