All that training is irrelevant now that you’re dead

The veterans know who the enemies are and keeps their ego in check. Whether you’re old school or new school doesn’t matter, DOOM still holds the crown (the hat). This ties in with next line about the fitted.

There was in fact a drive-by

Confident before the massacre

Now you’re calling for backup but it’s too late

Should have quit before the situation got to this point

Soldiers wear dogtags to identify families of the death if the body can’t be extracted or if the body is damaged to an extent that you can’t tell who it was. DOOM uses them to set up booby traps

Those hit by the explosion die instantly while those still surviving the first part of the ambust die a slower and more painful death, or in a shower of bullets.

The last thing you see before you die is the explosion

DOOM is ill, and after explosions you body goes limb for a few moments due to shock