No doubt, killing monsters is the deal. Overkill really only matters if you're going to use DPR as a metric, because it adjusts DPR downward, universally. DPR isn't interested much in smart tactics, at least, not in any presentation I've seen, only maximizing damage through build choices. What considering overkill does is says that a PC with a lower DPR might be equally as effective, especially over the last few points. Heck, I showed above that there's a range where GWM doesn't actually outperform no GWM with the same build choice. DPR calculations show that the GWM build significantly outpaces the non-GWM build, but it turns out that's not always, or even mostly, true. It mildly outpaces.
If you're not interested in DPR as a metric, though, then overkill doesn't matter either.