Slaves were essentially property, worth money and were used for extremely cheap labor. Killing the slave would essentially be purposely crashing your car. That is why slavers practiced cruel punishments such as beatings to discipline disobedient slaves.
The most straightforward theory for Western Rome's collapse pins the fall on a string of military losses sustained against outside forces. Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the Empire's borders.
