It will be up to the lower court to decide whether Giles killed his ex-girlfriend so that she wouldn't testify against him. However that issue is decided, the case against Giles seems strong, and he'll likely be convicted in a new trial even without the hearsay.

The decision suggests that "dying declarations" may be an exception to the Confrontation Clause, but Giles's girlfriend wasn't dying when she talked to the officer.

Some will think the result unfair, but Justice Scalia makes a convincing argument to the contrary:

The common-law forfeiture rule was aimed at removing the otherwise powerful incentive for defendants to intimidate, bribe, and kill the witnesses against them—in other words, it is grounded in “the ability of courts to protect the integrity of their proceedings.” Davis, 547 U. S., at 834. The boundaries of the doctrine seem to us intelligently fixed so as to avoid a principle repugnant to our constitutional system of trial by jury: that those murder defendants whom the judge considers guilty (after less than a full trial, mind you, and of course before the jury has pronounced guilt) should be deprived of fair-trial rights, lest they benefit from their judge-determined wrong....

[T]he guarantee of confrontation is no guarantee at all if it is subject to whatever exceptions courts from time to time consider “fair.” It is not the role of courts to extrapolate from the words of the Sixth Amendment to the values behind it, and then to enforce its guarantees only to the extent they serve (in the courts’ views) those underlying values.

Justice Scalia also examined the dissent's view that the majority decision rewards perpetrators of domestic abuse:

Is the suggestion that we should have one Confrontation Clause (the one the Framers adopted and Crawford described) for all other crimes, but a special, improvised, Confrontation Clause for those crimes that are frequently directed against women?

Both the majority opinion and Justice Souter's concurring opinion leave open the possibility that an abusive relationship may provide evidence that the defendant intended to kill in order to prevent the decedent from testifying against him. As Justice Souter puts it:

[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.

Whether that suggestion makes sense will depend on the facts of the case. In any event, the decision is another victory for the Confrontation Clause, a right that was becoming increasingly meaningless until the Court revived it in Crawford.