Why are we not allowed to "politicize" this? Illegal immigration is a political issue. If this guy was an illegal immigrant, I think it is fair to point out that Mollie's death might have been prevented with stronger border enforcement. The same for Kate. The same for any number of folks killed or harmed in other ways by illegal immigrants, whether intentional or not.
The argument that these ladies and anyone else could have just as likely (or more likely) been killed or harmed by a citizen or legal resident rings hollow. Those folks have a right to be here. Illegal aliens do not.
Now if you want to argue that "sure illegal immigrants kill or harm Americans, but that's still not a reason for stronger border enforcement" that's fine. You're entitled to make that argument. You're not entitled to sweep these incidents under the rug.