Super Hydorah

To achieve this ending, you only need to beat the final level before destroying all 4 Hydorah seeds present. Easiest level to avoid is 'Path of Scylla' as it is a dead end world on your map, and doesn't need to be accessed to reach any other levels. Beating the game from here really comes down to what firepower are you comfortable taking into the final level. I ended up using the 4 'scort' + Laser + Turriwall combo and was able to skip quite a few levels in the end to achieve this 'bad ending', but other players may find weapon combos that work better for them.