Robert had succeeded in killing Damien in the end.

And, not only that, but he hadn't been chased the police, hadn't been shot in the church...

What do people think would have happened to him?

I think that Gregory Peck is too sure of character to portray someone who would then commit suicide for not only having killed Damien, but for having placed himself and Kathy in that scenario in the first place.

Fake his death and live quietly somewhere?

Maybe, but unlikely.

He'd definitely live out his days in grief and torment of a kind.

But, for me, the deaths of Kathy and the other victims are far more shocking than Robert's own demise. The only surprise is that it's the Diplomatic Protection Force that kill him; the section of the Met assigned to support Emanssy staff in protecting the Ambassador.

I guess it's the way that the camera lingers on Kathy's photo by the bed during Robert's final drive home and his almost unnatural focus in getting Damien to the church on time. Sort of like he's resigned to his own fate.

I guess, if Robert had succeeded and lived, it may have been fitting for him to end in the monastery in Suboacp, serving out his penance with Spiletto.