Death #1: The Turkish Diplomat
Death #2: Matthew Crawley
Seriously, moments after Mary gives birth, Matthew is dead. Matthew was not an accomplished driver. In 1921, cars required a great deal of focus to safely operate. Mary knows this and calculates that there would be a great likelihood of death if Matthew drives alone back to Downton after seeing his new son. Death is inevitable. The blood on Mary's hands grows darker.
Death #3: Lord Grantham
So, she's killed a couple people. Who cares?! It's the roaring '20s, everyone is a little guilty of murder these days. Edith cares. Edith knows. Edith waits.
Don't you dare give Edith an eye roll!