Not defending that montage but the lack of tension/gravity around the murders I saw as a deliberate stylistic choice to emphasise just how cold DeNeiro's character was. Murder isn't thrilling or exciting for him, it's just following orders. I thought the movie would be a lot more violent than it was tbh, there aren't many onscreen killings at all.The murders really bugged me. No tension or considering the gravity of that act. The montage where he throws loads of guns into the river struck me as so lazy. More guns = lots of murders!