This case arose in the domestic violence context, as this issue very often does. Justices Souter and Ginsburg, who cast the deciding votes, are clearly concerned about the difficulty of proving purpose and offer this observation:

[T]he element of intention would normally be satisfied by the intent inferred on the part of the domestic abuser in the classic abusive relationship, which is meant to isolate the victim from outside help, including the aid of law enforcement and the judicial process. If the evidence for admissibility shows a continuing relationship of this sort, it would make no sense to suggest that the oppressing defendant miraculously abandoned the dynamics of abuse the instant before he killed his victim, say in a fit of anger.

The majority opinion, perhaps to pick up these two votes, says at page 23 of the slip opinion:

The domestic-violence context is, however, relevant for a separate reason. Acts of domestic violence often are intended to dissuade a victim from resorting to outside help, and include conduct designed to prevent testimony to police officers or cooperation in criminal prosecutions.Where such an abusive relationship culminates in murder, the evidence may support a finding that the crime expressed the intent to isolate the victim and to stop her from reporting abuse to the authorities or cooperating with a criminal prosecution—rendering her prior statements admissible under the forfeiture doctrine. Earlier abuse, or threats of abuse, intended to dissuade the victim from resorting to outside help would be highly relevant to this inquiry, as would evidence of ongoing criminal proceedings at which the victim would have been expected to testify.

Note that the running head of the slip opinion on this page is incorrect. Part II-E is the Opinion of the Court, not Opinion of Scalia, J.

Another interesting point here is that five justices express some skepticism as to whether the murdered girlfriend's prior statement was "testimonial" at all, noting that the testimonial nature was merely assumed because it was not in the question presented. If it was not testimonial, the Confrontation Clause is inapplicable under Crawford, and the question becomes a straight state-law hearsay question. Justice Thomas's position on this is no surprise, given his dissent in the Hammon companion case in Davis v. Washington, 547 U.S. 813 (2006). Justice Alito joined Hammon, but he took the trouble to write separately in Giles and note, "I am not convinced that the out-of-court statement at issue here fell within the Confrontation Clause in the first place." The dissent notes, "It is important to underscore that this case is premised on the assumption, not challenged here, that the witness’ statements are testimonial for purposes of the Confrontation Clause."

So, a prosecutor with a case similar to Giles on the circumstances of the prior statement should not concede that it is "testimonial." The statement was one "made to a police officer responding to a domestic-violence report about three weeks before the shooting." It was apparently fresh after the assault, and the victim "was crying when she spoke." I probably would have concluded this was "testimonial" after Davis/Hammon, but maybe the Court will take a narrower view of that construct in the next case.