They were sent on a health and welfare check. For a health and welfare check it is assumed that the resident is home but may be incapacitated or need assistance.
This was not a no knock raid. The police had an obligation to identify themselves when they got there, not skulk around in the dark and then only identify themselves as they were pulling the trigger to murder the resident.
You're making an assumption. Yes the 911 call was for a welfare check, but did the two cops get that message? Apparently not. Both of them treated the scene as a possible burglary. And acted appropriately.