Had retreatment of two root canals...having heat sensitivity with one of the molars treated-shouldn't tooth be "dead" now, not sensitive?

You are right.: With any root canal treatment, especially a retreatment, there is no nerve remaining that can feel hot or cold sensitivity. There must be something else going on. You should have the surrounding teeth evaluated by your dentist.

Endo retreat pain: Root canal retreatments are about 60-80% successful. A flare-up--sensitivity to thermal changes or pain when biting--are very possible. If you were prescribed antibiotics and/or pain medication take them as directed. If pain persists return to your treating dentist for follow-up.

Unusual:get retested: Heat sensitivity in a tooth that has had a root canal is unusual. The heat sensitivity is most likely coming from another tooth. On rare occasion, there may be a missed canal with vital tissue in it. If the tooth is being retreated due to abscess it is even more likely to be from another tooth, because an abscessed tooth typically has a dead nerve and thus shouldn't have thermal sensitivity.