Dylan and Anna didn't care about these people.
They were walking on their way back to the base and Dylan could sense that Anna's mood was not quite right.
Even though Anna hated the Williams Family, she didn't exactly wish for their death. Not in the past and probably not now. What she wanted was for them to live without have any interaction whatsoever.
What was good from staining her own hands with their blood?
There was nothing good.
Dylan could feel the complicated feelings in Anna's heart, so he was the one who made the last move. This way, the one who did it directly was not her but him.
After all, for Dylan the Williams Family was more of a stranger, so he didn't have much feelings for them. Killing people who didn't have any emotional bonds with them was much easier compared to killing people whom he knew and close with.
Call him cold blooded or anything.