The crime seems to be a peculiar mix of preparedness and chaos.
Whatever his mission, THAT NIGHT he didn't abort. Wasn't detered by the police presence around his apartment, wasn't detered by the fog, wasn't detered by having his phone along, but something slowed him up once he got in the vicinity... made several passes. Whatever held him back didn't ultimately deter him.
I do think he mentally planned for quite some time. Had a murder weapon, had a target, ran reconnaissance. May have doctored his vehicle -- messed with the license plate -- lined the interior -- assembled a kill kit -- weapon/s, sheath/s, gloves, mask, overalls, etc.
I think D may have been right, that it WAS K who said they're was someone there -- perhaps Murphy reacted to the door dash delivery or to BK, and K moved to quiet him before he might start to bark, waking everyone up. Puts Murphy in her room, climbs up and over M who is 98% asleep, and yells her 'there's sometime here' -- not in alarm, by way of explaining Murphy stirring. Nothing to worry about, just someone's here.
Even possible that more was said, of which D only heard the part: there's someone here, but it's ok -- he (door dash) drove away.
If all the bedrooms had locks, why wasn't M's door locked? Why did BK assume he could get past locked doors?
Hot prowler? Was BK an accomplished lockpicker? DID he pick a lock?
With all his planning, it would seem he failed fullbore to account for moving parts, namely the movements of people within the house. Dynamic environment... IMO he simply failed to anticipate double occupancy in M's bed, awake subjects within the home. 4 am may have seemed like a safe bet....
One wonders what stopped him from entering on the previous passes...
All his countermeasures for naught when he left his DNA on the bed (in the snap well of his Kbar sheath)...
So maybe he was wearing fillet gloves and that's why he was able to leave the house without realizing he had no sheath, he was double-protected, by hilt and glove.
Or... BK brought backup. TWO Kbar knives, TWO sheaths -- and left the house with two knives holstered in one sheath.
JMO