Question

what if the fugitive slave act was enforced with the death penalty? instead of just sending the slaves back?

Answer 1

It would cause slaves to be more weary of capture and those helping slaves to be more secretive.

It would also be harmful to the slave master because he would be losing his property. The slave master wants fugitive slaves back because he wishes to gain back the value he would have lost given that the slave had successfully escaped. Killing the slave would be just as bad for the slave master. The only benefit would be that slaves would be more reluctant to run away.

Making the punishment for fugitive slaves would harm both the slave and the master.