Dad grew up on a farm.
If your farm animals make any noise at night - they are being upset by something. A hungry predator in the hen house, pig yard, or barn could mean a hungry farm family that winter (or a broke one if those were the "cash crop" and they lived in a time & place when subsistence farming was no longer the rule). Upset dairy animals dry up and there goes the milk, butter, and cheese.......again - either your food or your "cash crop".
So the farmer was wise to run out to see what was going on - and unlucky enough that the slave slew him. If it had been a bear, a wolf, or a human predator unknown to him, he could still have ended up dead - but probably recognized the slave and thought that the slave was there to "help" him instead of helping himself to the pig......