^ To expand on point #1, most mass shooters either commit suicide as their final act, or fully expect that the police may put them down during the act. Mass shootings are therefore under the umbrella of suicide methods. Threatening a suicidal person with the death penalty makes zero sense.
This seems to be the historical case, but is the trend in the other direction? More mass shootings now end with the perp being taken into custody. Dylan Roof and Nikolas Cruz stand out as obvious examples, while I can't even think of a single notable event from the 1990s or 2000s where this was the case.
Perhaps because mass shooters, with associated 'manifestos' can now gather a sympathetic ear from those they are trying to appeal to.
