When modifying Doom sprites, you can't insert extra frames of animation. The frame rate is what it is, and you have to live with it. I used to make Doom mods all of the time, so I know how hard it is to create decent-looking death animations. The trick is to make sure there is that constant flow. Almost everything should  continue animating and nothing should remain too stagnant.  Keep an eye out for things that don't move. For example: Those spikes on our left should stay animating during their drop, but there is a frame where they just kind of stop, then continue animating. Also, the brain becomes exposed at the top of the head, but just disappears in a single frame. I understand that it's dropping out, but even adding the slightest quiver to show some stress would help a lot.